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Updated: Thursday, June 13, 2019
 
 

libpng (3)

Name

libpng - Portable Network Graphics (PNG) Reference Library 1.4.20

Synopsis

#include <png.h>

png_uint_32 png_access_version_number (void);

void png_benign_errors (png_structp png_ptr, png_const_charp error);

void png_build_grayscale_palette (int bit_depth, png_colorp palette);

png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
error);

void png_chunk_error (png_structp png_ptr, png_const_charp error);

void png_chunk_warning (png_structp png_ptr, png_const_charp message);

void png_convert_from_struct_tm  (png_timep  ptime,  struct  tm  FAR  *
ttime);

void png_convert_from_time_t (png_timep ptime, time_t ttime);

png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
ptime);

png_infop png_create_info_struct (png_structp png_ptr);

png_structp   png_create_read_struct   (png_const_charp   user_png_ver,
png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

png_structp   png_create_read_struct_2  (png_const_charp  user_png_ver,
png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

png_structp   png_create_write_struct   (png_const_charp  user_png_ver,
png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

png_structp  png_create_write_struct_2  (png_const_charp  user_png_ver,
png_voidp  error_ptr,  png_error_ptr  error_fn,  png_error_ptr warn_fn,
png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

void  png_data_freer  (png_structp  png_ptr,  png_infop  info_ptr,  int
freer, png_uint_32 mask));

void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
info_ptr_ptr);

void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
info_ptr_ptr, png_infopp end_info_ptr_ptr);

void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
info_ptr_ptr);

void png_err (png_structp png_ptr);

void png_error (png_structp png_ptr, png_const_charp error);

void png_free (png_structp png_ptr, png_voidp ptr);

void png_free_chunk_list (png_structp png_ptr);

void png_free_default (png_structp png_ptr, png_voidp ptr);

void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
info_ptr);

png_uint_32    png_get_bKGD   (png_const_structp   png_ptr,   png_infop
info_ptr, png_color_16p *background);

png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
info_ptr);

png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
info_ptr, double  *white_x,  double  *white_y,  double  *red_x,  double
*red_y,  double  *green_x,  double  *green_y,  double  *blue_x,  double
*blue_y);

png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
png_const_infop  info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
info_ptr);

png_uint_32     png_get_compression_buffer_size      (png_const_structp
png_ptr);

png_byte     png_get_compression_type    (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_byte png_get_copyright (png_const_structp png_ptr);

png_voidp png_get_error_ptr (png_const_structp png_ptr);

png_byte      png_get_filter_type      (png_const_structp      png_ptr,
png_const_infop info_ptr);

png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
info_ptr, double *file_gamma);

png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
png_const_infop info_ptr, png_uint_32 *int_file_gamma);

png_byte png_get_header_ver (png_const_structp png_ptr);

png_byte png_get_header_version (png_const_structp png_ptr);

png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
info_ptr, png_uint_16p *hist);

png_uint_32 png_get_iCCP  (png_const_structp  png_ptr,  png_const_infop
info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
png_uint_32 *proflen);

png_uint_32  png_get_IHDR  (png_structp  png_ptr,  png_infop  info_ptr,
png_uint_32   *width,   png_uint_32   *height,   int   *bit_depth,  int
*color_type, int  *interlace_type,  int  *compression_type,  int  *fil-
ter_type);

png_uint_32     png_get_image_height     (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
png_const_infop info_ptr);

png_int_32 png_get_int_32 (png_bytep buf);

png_byte     png_get_interlace_type     (png_const_structp     png_ptr,
png_const_infop info_ptr);

png_const_bytep png_get_io_chunk_name (png_structp png_ptr);

png_voidp png_get_io_ptr (png_structp png_ptr);

png_uint_32 png_get_io_state (png_structp png_ptr);

png_byte png_get_libpng_ver (png_const_structp png_ptr);

png_voidp png_get_mem_ptr (png_const_structp png_ptr);

png_uint_32 png_get_oFFs  (png_const_structp  png_ptr,  png_const_infop
info_ptr,    png_uint_32    *offset_x,   png_uint_32   *offset_y,   int
*unit_type);

png_uint_32 png_get_pCAL  (png_const_structp  png_ptr,  png_const_infop
info_ptr,  png_charp  *purpose,  png_int_32  *X0,  png_int_32  *X1, int
*type, int *nparams, png_charp *units, png_charpp *params);

png_uint_32 png_get_pHYs  (png_const_structp  png_ptr,  png_const_infop
info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

float     png_get_pixel_aspect_ratio     (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_uint_32      png_get_pHYs_dpi      (png_const_structp      png_ptr,
png_const_infop  info_ptr,  png_uint_32 *res_x, png_uint_32 *res_y, int
*unit_type);

png_fixed_point   png_get_pixel_aspect_ratio_fixed   (png_const_structp
png_ptr, png_const_infop info_ptr);

png_uint_32    png_get_pixels_per_inch    (png_const_structp   png_ptr,
png_const_infop info_ptr);

png_uint_32   png_get_pixels_per_meter   (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
info_ptr, png_colorp *palette, int *num_palette);

png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

png_uint_32      png_get_rowbytes      (png_const_structp      png_ptr,
png_const_infop info_ptr);

png_bytepp  png_get_rows  (png_const_structp  png_ptr,  png_const_infop
info_ptr);

png_uint_32   png_get_sBIT   (png_const_structp   png_ptr,    png_infop
info_ptr, png_color_8p *sig_bit);

void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
int* unit, double* width, double* height);

void  png_get_sCAL_fixed  (png_const_structp  png_ptr,  png_const_infop
info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
info_ptr, int* unit, png_charpp width, png_charpp height);

png_bytep  png_get_signature  (png_const_structp   png_ptr,   png_infop
info_ptr);

png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
info_ptr, png_spalette_p *splt_ptr);

png_uint_32 png_get_sRGB  (png_const_structp  png_ptr,  png_const_infop
info_ptr, int *intent);

png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
info_ptr, png_textp *text_ptr, int *num_text);

png_uint_32   png_get_tIME   (png_const_structp   png_ptr,    png_infop
info_ptr, png_timep *mod_time);

png_uint_32    png_get_tRNS   (png_const_structp   png_ptr,   png_infop
info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,   png_color_16p
*trans_color);

/*   This   function   is   really  an  inline  macro.  */  png_uint_16
png_get_uint_16 (png_bytep buf);

png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

/*  This  function  is  really  an   inline   macro.   */   png_uint_32
png_get_uint_32 (png_bytep buf);

png_uint_32    png_get_unknown_chunks    (png_const_structp    png_ptr,
png_const_infop info_ptr, png_unknown_chunkpp unknowns);

png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
info_ptr, png_uint_32 flag);

float      png_get_x_offset_inches      (png_const_structp     png_ptr,
png_const_infop info_ptr);

png_fixed_point  png_get_x_offset_inches_fixed  (png_structp   png_ptr,
png_const_infop info_ptr);

png_int_32    png_get_x_offset_microns    (png_const_structp   png_ptr,
png_const_infop info_ptr);

png_int_32    png_get_x_offset_pixels    (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_uint_32   png_get_x_pixels_per_inch   (png_const_structp   png_ptr,
png_const_infop info_ptr);

png_uint_32  png_get_x_pixels_per_meter   (png_const_structp   png_ptr,
png_const_infop info_ptr);

float      png_get_y_offset_inches      (png_const_structp     png_ptr,
png_const_infop info_ptr);

png_fixed_point  png_get_y_offset_inches_fixed  (png_structp   png_ptr,
png_const_infop info_ptr);

png_int_32    png_get_y_offset_microns    (png_const_structp   png_ptr,
png_const_infop info_ptr);

png_int_32    png_get_y_offset_pixels    (png_const_structp    png_ptr,
png_const_infop info_ptr);

png_uint_32   png_get_y_pixels_per_inch   (png_const_structp   png_ptr,
png_const_infop info_ptr);

png_uint_32  png_get_y_pixels_per_meter   (png_const_structp   png_ptr,
png_const_infop info_ptr);

int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

void      png_info_init_3      (png_infopp     info_ptr,     png_size_t
png_info_struct_size);

void png_init_io (png_structp png_ptr, FILE *fp);

png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
size);

png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

png_uint_32  png_permit_mng_features  (png_structp png_ptr, png_uint_32
mng_features_permitted);

void  png_process_data  (png_structp   png_ptr,   png_infop   info_ptr,
png_bytep buffer, png_size_t buffer_size);

void   png_progressive_combine_row   (png_structp   png_ptr,  png_bytep
old_row, png_bytep new_row);

void png_read_end (png_structp png_ptr, png_infop info_ptr);

void png_read_image (png_structp png_ptr, png_bytepp image);

void png_read_info (png_structp png_ptr, png_infop info_ptr);

void png_read_png (png_structp png_ptr, png_infop info_ptr, int  trans-
forms, png_voidp params);

void  png_read_row  (png_structp png_ptr, png_bytep row, png_bytep dis-
play_row);

void png_read_rows (png_structp  png_ptr,  png_bytepp  row,  png_bytepp
display_row, png_uint_32 num_rows);

void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

int png_reset_zstream (png_structp png_ptr);

void png_save_int_32 (png_bytep buf, png_int_32 i);

void png_save_uint_16 (png_bytep buf, unsigned int i);

void png_save_uint_32 (png_bytep buf, png_uint_32 i);

void  png_set_add_alpha  (png_structp  png_ptr, png_uint_32 filler, int
flags);

void  png_set_background  (png_structp  png_ptr,  png_color_16p   back-
ground_color,  int background_gamma_code, int need_expand, double back-
ground_gamma);

void png_set_background_fixed (png_structp png_ptr, png_color_16p back-
ground_color,  int  background_gamma_code, int need_expand, png_uint_32
background_gamma);

void png_set_benign_errors (png_structp png_ptr, int allowed);

void png_set_bgr (png_structp png_ptr);

void   png_set_bKGD   (png_structp   png_ptr,    png_infop    info_ptr,
png_color_16p background);

void  png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr, double
white_x, double white_y, double red_x, double  red_y,  double  green_x,
double green_y, double blue_x, double blue_y);

void   png_set_cHRM_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
png_uint_32   white_x,   png_uint_32   white_y,   png_uint_32    red_x,
png_uint_32    red_y,   png_uint_32   green_x,   png_uint_32   green_y,
png_uint_32 blue_x, png_uint_32 blue_y);

void   png_set_chunk_cache_max   (png_structp   png_ptr,    png_uint_32
user_chunk_cache_max);

void png_set_compression_level (png_structp png_ptr, int level);

void    png_set_compression_mem_level    (png_structp    png_ptr,   int
mem_level);

void png_set_compression_method (png_structp png_ptr, int method);

void png_set_compression_strategy (png_structp png_ptr, int strategy);

void png_set_compression_window_bits  (png_structp  png_ptr,  int  win-
dow_bits);

void  png_set_crc_action  (png_structp  png_ptr,  int  crit_action, int
ancil_action);

void  png_set_error_fn  (png_structp  png_ptr,   png_voidp   error_ptr,
png_error_ptr error_fn, png_error_ptr warning_fn);

void png_set_expand (png_structp png_ptr);

void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
flags);

void png_set_filter (png_structp png_ptr, int method, int filters);

void  png_set_filter_heuristics  (png_structp  png_ptr,   int   heuris-
tic_method,  int  num_weights,  png_doublep filter_weights, png_doublep
filter_costs);

void png_set_filter_heuristics_fixed (png_structp png_ptr, int  heuris-
tic_method,    int   num_weights,   png_fixed_point_p   filter_weights,
png_fixed_point_p filter_costs);

void png_set_flush (png_structp png_ptr, int nrows);

void png_set_gamma (png_structp png_ptr,  double  screen_gamma,  double
default_file_gamma);

void     png_set_gamma_fixed    (png_structp    png_ptr,    png_uint_32
screen_gamma,  png_uint_32   default_file_gamma);   void   png_set_gAMA
(png_structp png_ptr, png_infop info_ptr, double file_gamma);

void   png_set_gAMA_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
png_uint_32 file_gamma);

void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

void png_set_gray_to_rgb (png_structp png_ptr);

void   png_set_hIST   (png_structp   png_ptr,    png_infop    info_ptr,
png_uint_16p hist);

void    png_set_iCCP    (png_structp   png_ptr,   png_infop   info_ptr,
png_const_charp name, int  compression_type,  png_const_bytep  profile,
png_uint_32 proflen);

int png_set_interlace_handling (png_structp png_ptr);

void  png_set_invalid  (png_structp  png_ptr,  png_infop  info_ptr, int
mask);

void png_set_invert_alpha (png_structp png_ptr);

void png_set_invert_mono (png_structp png_ptr);

void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
width,  png_uint_32  height,  int bit_depth, int color_type, int inter-
lace_type, int compression_type, int filter_type);

void  png_set_keep_unknown_chunks  (png_structp  png_ptr,   int   keep,
png_bytep chunk_list, int num_chunks);

jmp_buf*   png_set_longjmp_fn   (png_structp  png_ptr,  png_longjmp_ptr
longjmp_fn, size_t jmp_buf_size);

void png_set_chunk_malloc_max  (png_structp  png_ptr,  png_alloc_size_t
user_chunk_cache_max);

void  png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
size);

void png_set_mem_fn (png_structp png_ptr, png_voidp  mem_ptr,  png_mal-
loc_ptr malloc_fn, png_free_ptr free_fn);

void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
offset_x, png_uint_32 offset_y, int unit_type);

void png_set_packing (png_structp png_ptr);

void png_set_packswap (png_structp png_ptr);

void png_set_palette_to_rgb (png_structp png_ptr);

void png_set_pCAL (png_structp png_ptr, png_infop  info_ptr,  png_charp
purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
units, png_charpp params);

void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
res_x, png_uint_32 res_y, int unit_type);

void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro-
gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
row_fn, png_progressive_end_ptr end_fn);

void  png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
palette, int num_palette);

void png_set_quantize (png_structp  png_ptr,  png_colorp  palette,  int
num_palette, int maximum_colors, png_uint_16p histogram, int full_quan-
tize);

void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
read_data_fn);

void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
read_row_fn);

void   png_set_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp
user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

void      png_set_read_user_transform_fn      (png_structp     png_ptr,
png_user_transform_ptr read_user_transform_fn);

void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
red, double green);

void  png_set_rgb_to_gray_fixed  (png_structp png_ptr, int error_action
png_uint_32 red, png_uint_32 green);

void png_set_rows (png_structp png_ptr, png_infop info_ptr,  png_bytepp
row_pointers);

void    png_set_sBIT    (png_structp   png_ptr,   png_infop   info_ptr,
png_color_8p sig_bit);

void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,  int  unit,
double width, double height);

void  png_set_sCAL_fixed  (png_structp png_ptr, png_infop info_ptr, int
unit, png_fixed_point width, png_fixed_point height);

void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
png_charp  width,  png_charp  height);  void png_set_shift (png_structp
png_ptr, png_color_8p true_bits);

void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

void   png_set_sPLT   (png_structp   png_ptr,    png_infop    info_ptr,
png_spalette_p splt_ptr, int num_spalettes);

void   png_set_sRGB   (png_structp  png_ptr,  png_infop  info_ptr,  int
intent);

void   png_set_sRGB_gAMA_and_cHRM   (png_structp   png_ptr,   png_infop
info_ptr, int intent);

void png_set_strip_16 (png_structp png_ptr);

void png_set_strip_alpha (png_structp png_ptr);

void   png_set_strip_error_numbers  (png_structp  png_ptr,  png_uint_32
strip_mode);

void png_set_swap (png_structp png_ptr);

void png_set_swap_alpha (png_structp png_ptr);

void png_set_text (png_structp png_ptr, png_infop  info_ptr,  png_textp
text_ptr, int num_text);

void  png_set_tIME  (png_structp png_ptr, png_infop info_ptr, png_timep
mod_time);

void png_set_tRNS (png_structp png_ptr, png_infop  info_ptr,  png_bytep
trans_alpha, int num_trans, png_color_16p trans_color);

void png_set_tRNS_to_alpha (png_structp png_ptr);

png_uint_32   png_set_unknown_chunks  (png_structp  png_ptr,  png_infop
info_ptr, png_unknown_chunkp unknowns, int num, int location);

void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
info_ptr, int chunk, int location);

void     png_set_user_limits    (png_structp    png_ptr,    png_uint_32
user_width_max, png_uint_32 user_height_max);

void  png_set_user_transform_info   (png_structp   png_ptr,   png_voidp
user_transform_ptr,  int user_transform_depth, int user_transform_chan-
nels);

void   png_set_write_fn   (png_structp   png_ptr,   png_voidp   io_ptr,
png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
write_row_fn);

void     png_set_write_user_transform_fn     (png_structp      png_ptr,
png_user_transform_ptr write_user_transform_fn);

int   png_sig_cmp   (png_bytep   sig,   png_size_t   start,  png_size_t
num_to_check);

void png_start_read_image (png_structp png_ptr);

void png_warning (png_structp png_ptr, png_const_charp message);

void  png_write_chunk  (png_structp  png_ptr,   png_bytep   chunk_name,
png_bytep data, png_size_t length);

void   png_write_chunk_data   (png_structp   png_ptr,  png_bytep  data,
png_size_t length);

void png_write_chunk_end (png_structp png_ptr);

void png_write_chunk_start (png_structp png_ptr, png_bytep  chunk_name,
png_uint_32 length);

void png_write_end (png_structp png_ptr, png_infop info_ptr);

void png_write_flush (png_structp png_ptr);

void png_write_image (png_structp png_ptr, png_bytepp image);

void png_write_info (png_structp png_ptr, png_infop info_ptr);

void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
info_ptr);

void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans-
forms, png_voidp params);

void png_write_row (png_structp png_ptr, png_bytep row);

void  png_write_rows  (png_structp png_ptr, png_bytepp row, png_uint_32
num_rows);

void png_write_sig (png_structp png_ptr);

Description

LIBPNG(3)                  Library Functions Manual                  LIBPNG(3)



NAME
       libpng - Portable Network Graphics (PNG) Reference Library 1.4.20

SYNOPSIS
        #include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_errors (png_structp png_ptr, png_const_charp error);

       void png_build_grayscale_palette (int bit_depth, png_colorp palette);

       png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

       void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

       void png_convert_from_struct_tm  (png_timep  ptime,  struct  tm  FAR  *
       ttime);

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

       png_structp   png_create_read_struct   (png_const_charp   user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp   png_create_read_struct_2  (png_const_charp  user_png_ver,
       png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       png_structp   png_create_write_struct   (png_const_charp  user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp  png_create_write_struct_2  (png_const_charp  user_png_ver,
       png_voidp  error_ptr,  png_error_ptr  error_fn,  png_error_ptr warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       void  png_data_freer  (png_structp  png_ptr,  png_infop  info_ptr,  int
       freer, png_uint_32 mask));

       void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
       info_ptr_ptr);

       void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
       info_ptr_ptr, png_infopp end_info_ptr_ptr);

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_err (png_structp png_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default (png_structp png_ptr, png_voidp ptr);

       void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

       png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
       info_ptr);

       png_uint_32    png_get_bKGD   (png_const_structp   png_ptr,   png_infop
       info_ptr, png_color_16p *background);

       png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double  *white_x,  double  *white_y,  double  *red_x,  double
       *red_y,  double  *green_x,  double  *green_y,  double  *blue_x,  double
       *blue_y);

       png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
       png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
       png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

       png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

       png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

       png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
       info_ptr);

       png_uint_32     png_get_compression_buffer_size      (png_const_structp
       png_ptr);

       png_byte     png_get_compression_type    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_byte png_get_copyright (png_const_structp png_ptr);

       png_voidp png_get_error_ptr (png_const_structp png_ptr);

       png_byte      png_get_filter_type      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double *file_gamma);

       png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
       png_const_infop info_ptr, png_uint_32 *int_file_gamma);

       png_byte png_get_header_ver (png_const_structp png_ptr);

       png_byte png_get_header_version (png_const_structp png_ptr);

       png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
       png_uint_32 *proflen);

       png_uint_32  png_get_IHDR  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   *width,   png_uint_32   *height,   int   *bit_depth,  int
       *color_type, int  *interlace_type,  int  *compression_type,  int  *fil-
       ter_type);

       png_uint_32     png_get_image_height     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       png_byte     png_get_interlace_type     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_const_bytep png_get_io_chunk_name (png_structp png_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_uint_32 png_get_io_state (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_const_structp png_ptr);

       png_voidp png_get_mem_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_oFFs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,    png_uint_32    *offset_x,   png_uint_32   *offset_y,   int
       *unit_type);

       png_uint_32 png_get_pCAL  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charp  *purpose,  png_int_32  *X0,  png_int_32  *X1, int
       *type, int *nparams, png_charp *units, png_charpp *params);

       png_uint_32 png_get_pHYs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       float     png_get_pixel_aspect_ratio     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32      png_get_pHYs_dpi      (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *res_x, png_uint_32 *res_y, int
       *unit_type);

       png_fixed_point   png_get_pixel_aspect_ratio_fixed   (png_const_structp
       png_ptr, png_const_infop info_ptr);

       png_uint_32    png_get_pixels_per_inch    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_pixels_per_meter   (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_colorp *palette, int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

       png_uint_32      png_get_rowbytes      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_bytepp  png_get_rows  (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32   png_get_sBIT   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_color_8p *sig_bit);

       void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
       int* unit, double* width, double* height);

       void  png_get_sCAL_fixed  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

       void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
       info_ptr, int* unit, png_charpp width, png_charpp height);

       png_bytep  png_get_signature  (png_const_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_spalette_p *splt_ptr);

       png_uint_32 png_get_sRGB  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int *intent);

       png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_textp *text_ptr, int *num_text);

       png_uint_32   png_get_tIME   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_timep *mod_time);

       png_uint_32    png_get_tRNS   (png_const_structp   png_ptr,   png_infop
       info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,   png_color_16p
       *trans_color);

       /*   This   function   is   really  an  inline  macro.  */  png_uint_16
       png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

       /*  This  function  is  really  an   inline   macro.   */   png_uint_32
       png_get_uint_32 (png_bytep buf);

       png_uint_32    png_get_unknown_chunks    (png_const_structp    png_ptr,
       png_const_infop info_ptr, png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

       png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 flag);

       float      png_get_x_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_x_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_x_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_x_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       float      png_get_y_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_y_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_y_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_y_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void      png_info_init_3      (png_infopp     info_ptr,     png_size_t
       png_info_struct_size);

       void png_init_io (png_structp png_ptr, FILE *fp);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
       size);

       png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

       png_uint_32  png_permit_mng_features  (png_structp png_ptr, png_uint_32
       mng_features_permitted);

       void  png_process_data  (png_structp   png_ptr,   png_infop   info_ptr,
       png_bytep buffer, png_size_t buffer_size);

       void   png_progressive_combine_row   (png_structp   png_ptr,  png_bytep
       old_row, png_bytep new_row);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void png_read_png (png_structp png_ptr, png_infop info_ptr, int  trans-
       forms, png_voidp params);

       void  png_read_row  (png_structp png_ptr, png_bytep row, png_bytep dis-
       play_row);

       void png_read_rows (png_structp  png_ptr,  png_bytepp  row,  png_bytepp
       display_row, png_uint_32 num_rows);

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       int png_reset_zstream (png_structp png_ptr);

       void png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

       void  png_set_add_alpha  (png_structp  png_ptr, png_uint_32 filler, int
       flags);

       void  png_set_background  (png_structp  png_ptr,  png_color_16p   back-
       ground_color,  int background_gamma_code, int need_expand, double back-
       ground_gamma);

       void png_set_background_fixed (png_structp png_ptr, png_color_16p back-
       ground_color,  int  background_gamma_code, int need_expand, png_uint_32
       background_gamma);

       void png_set_benign_errors (png_structp png_ptr, int allowed);

       void png_set_bgr (png_structp png_ptr);

       void   png_set_bKGD   (png_structp   png_ptr,    png_infop    info_ptr,
       png_color_16p background);

       void  png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr, double
       white_x, double white_y, double red_x, double  red_y,  double  green_x,
       double green_y, double blue_x, double blue_y);

       void   png_set_cHRM_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   white_x,   png_uint_32   white_y,   png_uint_32    red_x,
       png_uint_32    red_y,   png_uint_32   green_x,   png_uint_32   green_y,
       png_uint_32 blue_x, png_uint_32 blue_y);

       void   png_set_chunk_cache_max   (png_structp   png_ptr,    png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void    png_set_compression_mem_level    (png_structp    png_ptr,   int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void png_set_compression_window_bits  (png_structp  png_ptr,  int  win-
       dow_bits);

       void  png_set_crc_action  (png_structp  png_ptr,  int  crit_action, int
       ancil_action);

       void  png_set_error_fn  (png_structp  png_ptr,   png_voidp   error_ptr,
       png_error_ptr error_fn, png_error_ptr warning_fn);

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

       void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
       flags);

       void png_set_filter (png_structp png_ptr, int method, int filters);

       void  png_set_filter_heuristics  (png_structp  png_ptr,   int   heuris-
       tic_method,  int  num_weights,  png_doublep filter_weights, png_doublep
       filter_costs);

       void png_set_filter_heuristics_fixed (png_structp png_ptr, int  heuris-
       tic_method,    int   num_weights,   png_fixed_point_p   filter_weights,
       png_fixed_point_p filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

       void png_set_gamma (png_structp png_ptr,  double  screen_gamma,  double
       default_file_gamma);

       void     png_set_gamma_fixed    (png_structp    png_ptr,    png_uint_32
       screen_gamma,  png_uint_32   default_file_gamma);   void   png_set_gAMA
       (png_structp png_ptr, png_infop info_ptr, double file_gamma);

       void   png_set_gAMA_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32 file_gamma);

       void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

       void   png_set_hIST   (png_structp   png_ptr,    png_infop    info_ptr,
       png_uint_16p hist);

       void    png_set_iCCP    (png_structp   png_ptr,   png_infop   info_ptr,
       png_const_charp name, int  compression_type,  png_const_bytep  profile,
       png_uint_32 proflen);

       int png_set_interlace_handling (png_structp png_ptr);

       void  png_set_invalid  (png_structp  png_ptr,  png_infop  info_ptr, int
       mask);

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

       void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
       width,  png_uint_32  height,  int bit_depth, int color_type, int inter-
       lace_type, int compression_type, int filter_type);

       void  png_set_keep_unknown_chunks  (png_structp  png_ptr,   int   keep,
       png_bytep chunk_list, int num_chunks);

       jmp_buf*   png_set_longjmp_fn   (png_structp  png_ptr,  png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void png_set_chunk_malloc_max  (png_structp  png_ptr,  png_alloc_size_t
       user_chunk_cache_max);

       void  png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
       size);

       void png_set_mem_fn (png_structp png_ptr, png_voidp  mem_ptr,  png_mal-
       loc_ptr malloc_fn, png_free_ptr free_fn);

       void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       offset_x, png_uint_32 offset_y, int unit_type);

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb (png_structp png_ptr);

       void png_set_pCAL (png_structp png_ptr, png_infop  info_ptr,  png_charp
       purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
       units, png_charpp params);

       void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       res_x, png_uint_32 res_y, int unit_type);

       void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro-
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

       void  png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
       palette, int num_palette);

       void png_set_quantize (png_structp  png_ptr,  png_colorp  palette,  int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan-
       tize);

       void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void   png_set_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp
       user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

       void      png_set_read_user_transform_fn      (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);

       void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
       red, double green);

       void  png_set_rgb_to_gray_fixed  (png_structp png_ptr, int error_action
       png_uint_32 red, png_uint_32 green);

       void png_set_rows (png_structp png_ptr, png_infop info_ptr,  png_bytepp
       row_pointers);

       void    png_set_sBIT    (png_structp   png_ptr,   png_infop   info_ptr,
       png_color_8p sig_bit);

       void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,  int  unit,
       double width, double height);

       void  png_set_sCAL_fixed  (png_structp png_ptr, png_infop info_ptr, int
       unit, png_fixed_point width, png_fixed_point height);

       void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
       png_charp  width,  png_charp  height);  void png_set_shift (png_structp
       png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

       void   png_set_sPLT   (png_structp   png_ptr,    png_infop    info_ptr,
       png_spalette_p splt_ptr, int num_spalettes);

       void   png_set_sRGB   (png_structp  png_ptr,  png_infop  info_ptr,  int
       intent);

       void   png_set_sRGB_gAMA_and_cHRM   (png_structp   png_ptr,   png_infop
       info_ptr, int intent);

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void   png_set_strip_error_numbers  (png_structp  png_ptr,  png_uint_32
       strip_mode);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

       void png_set_text (png_structp png_ptr, png_infop  info_ptr,  png_textp
       text_ptr, int num_text);

       void  png_set_tIME  (png_structp png_ptr, png_infop info_ptr, png_timep
       mod_time);

       void png_set_tRNS (png_structp png_ptr, png_infop  info_ptr,  png_bytep
       trans_alpha, int num_trans, png_color_16p trans_color);

       void png_set_tRNS_to_alpha (png_structp png_ptr);

       png_uint_32   png_set_unknown_chunks  (png_structp  png_ptr,  png_infop
       info_ptr, png_unknown_chunkp unknowns, int num, int location);

       void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
       info_ptr, int chunk, int location);

       void     png_set_user_limits    (png_structp    png_ptr,    png_uint_32
       user_width_max, png_uint_32 user_height_max);

       void  png_set_user_transform_info   (png_structp   png_ptr,   png_voidp
       user_transform_ptr,  int user_transform_depth, int user_transform_chan-
       nels);

       void   png_set_write_fn   (png_structp   png_ptr,   png_voidp   io_ptr,
       png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void     png_set_write_user_transform_fn     (png_structp      png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       int   png_sig_cmp   (png_bytep   sig,   png_size_t   start,  png_size_t
       num_to_check);

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

       void  png_write_chunk  (png_structp  png_ptr,   png_bytep   chunk_name,
       png_bytep data, png_size_t length);

       void   png_write_chunk_data   (png_structp   png_ptr,  png_bytep  data,
       png_size_t length);

       void png_write_chunk_end (png_structp png_ptr);

       void png_write_chunk_start (png_structp png_ptr, png_bytep  chunk_name,
       png_uint_32 length);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

       void  png_write_rows  (png_structp png_ptr, png_bytepp row, png_uint_32
       num_rows);

       void png_write_sig (png_structp png_ptr);


DESCRIPTION
       The libpng library supports encoding, decoding, and  various  manipula-
       tions  of  the  Portable Network Graphics (PNG) format image files.  It
       uses the zlib(3) compression library.   Following  is  a  copy  of  the
       libpng-manual.txt file that accompanies libpng.

LIBPNG.TXT
       libpng.txt - A description on how to use and modify libpng

        libpng version 1.4.20 - December 29, 2016
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2014 Glenn Randers-Pehrson

        This document is released under the libpng license.
        For conditions of distribution and use, see the disclaimer
        and license in png.h

        Based on:

        libpng versions 0.97, January 1998, through 1.4.20 - December 29, 2016
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2014 Glenn Randers-Pehrson

        libpng 1.0 beta 6  version 0.96 May 28, 1997
        Updated and distributed by Andreas Dilger
        Copyright (c) 1996, 1997 Andreas Dilger

        libpng 1.0 beta 2 - version 0.88  January 26, 1996
        For conditions of distribution and use, see copyright
        notice in png.h. Copyright (c) 1995, 1996 Guy Eric
        Schalnat, Group 42, Inc.

        Updated/rewritten per request in the libpng FAQ
        Copyright (c) 1995, 1996 Frank J. T. Wojcik
        December 18, 1995 & January 20, 1996


I. Introduction
       This  file  describes  how  to use and modify the PNG reference library
       (known as libpng) for your own use.  There are five  sections  to  this
       file:  introduction, structures, reading, writing, and modification and
       configuration notes for various special platforms.  In addition to this
       file,  example.c  is a good starting point for using the library, as it
       is heavily commented and should include  everything  most  people  will
       need.  We assume that libpng is already installed; see the INSTALL file
       for instructions on how to install libpng.

       For examples of libpng usage, see the files  "example.c",  "pngtest.c",
       and  the files in the "contrib" directory, all of which are included in
       the libpng distribution.

       Libpng was written as a companion to the PNG specification, as a way of
       reducing the amount of time and effort it takes to support the PNG file
       format in application programs.

       The PNG specification (second edition), November 2003, is available  as
       a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at
       <http://www.w3.org/TR/2003/REC-PNG-20031110/ The W3C and ISO  documents
       have identical technical content.

       The  PNG-1.2  specification  is  available  at  <http://png-mng.source-
       forge.net/pub/png/spec/1.2/>.  It is technically equivalent to the  PNG
       specification (second edition) but has some additional material.

       The  PNG-1.0  specification  is  available  as  RFC  2083  <http://png-
       mng.sourceforge.net/pub/png/spec/1.0/>  and  as  a  W3C  Recommendation
       <http://www.w3.org/TR/REC-png-961001>.

       Some  additional  chunks  are  described  in the special-purpose public
       chunks documents at <http://www.libpng.org/pub/png/spec/register/>

       Other information about PNG, and the latest version of libpng,  can  be
       found at the PNG home page, <http://www.libpng.org/pub/png/>.

       Most  users will not have to modify the library significantly; advanced
       users may want to modify it more.  All attempts were made to make it as
       complete  as possible, while keeping the code easy to understand.  Cur-
       rently, this library only supports C.  Support for other  languages  is
       being considered.

       Libpng has been designed to handle multiple sessions at one time, to be
       easily modifiable, to be portable to  the  vast  majority  of  machines
       (ANSI,  K&R,  16-,  32-,  and 64-bit) available, and to be easy to use.
       The ultimate goal of libpng is to promote the  acceptance  of  the  PNG
       file  format in whatever way possible.  While there is still work to be
       done (see the TODO file), libpng should cover the majority of the needs
       of its users.

       Libpng  uses  zlib  for its compression and decompression of PNG files.
       Further information about zlib, and the latest version of zlib, can  be
       found  at the zlib home page, <http://zlib.net/>.  The zlib compression
       utility is a general purpose utility that is useful for more  than  PNG
       files, and can be used without libpng.  See the documentation delivered
       with zlib for more details.  You can usually find the source files  for
       the zlib utility wherever you find the libpng source files.

       Libpng  is  thread  safe,  provided  the  threads  are  using different
       instances  of  the  structures.   Each  thread  should  have  its   own
       png_struct and png_info instances, and thus its own image.  Libpng does
       not protect itself against two threads using the  same  instance  of  a
       structure.


II. Structures
       There  are two main structures that are important to libpng, png_struct
       and png_info.  The first, png_struct, is  an  internal  structure  that
       will  not,  for  the  most  part, be used by a user except as the first
       variable passed to every libpng function call.

       The png_info structure is designed to provide information about the PNG
       file.  At one time, the fields of png_info were intended to be directly
       accessible to the user.  However, this tended to  cause  problems  with
       applications  using dynamically loaded libraries, and as a result a set
       of interface functions for png_info (the  png_get_*()  and  png_set_*()
       functions)  was  developed.  The fields of png_info are still available
       for older applications, but it is suggested that applications  use  the
       new interfaces if at all possible.

       Applications  that  do  make direct access to the members of png_struct
       (except for png_ptr->jmpbuf) must be recompiled whenever the library is
       updated,  and  applications  that  make direct access to the members of
       png_info must be recompiled if they were compiled or loaded with libpng
       version 1.0.6, in which the members were in a different order.  In ver-
       sion 1.0.7, the members of the png_info structure reverted to  the  old
       order,  as  they  were  in versions 0.97c through 1.0.5.  Starting with
       version 2.0.0, both structures are going to be hidden, and the contents
       of  the  structures will only be accessible through the png_get/png_set
       functions.

       The png.h header file is an invaluable reference for  programming  with
       libpng.   And  while I'm on the topic, make sure you include the libpng
       header file:

       #include <png.h>


III. Reading
       We'll now walk you through the possible functions to call when  reading
       in  a  PNG file sequentially, briefly explaining the syntax and purpose
       of each one.  See example.c and png.h for more detail.  While  progres-
       sive  reading  is covered in the next section, you will still need some
       of the functions discussed in this section to read a PNG file.


   Setup
       You will want to do the  I/O  initialization(*)  before  you  get  into
       libpng, so if it doesn't work, you don't have much to undo.  Of course,
       you will also want to insure that you are, in fact, dealing with a  PNG
       file.   Libpng  provides a simple check to see if a file is a PNG file.
       To use it, pass in the first 1 to 8 bytes of the file to  the  function
       png_sig_cmp(), and it will return 0 (false) if the bytes match the cor-
       responding bytes of the PNG signature, or nonzero (true) otherwise.  Of
       course,  the  more  bytes  you pass in, the greater the accuracy of the
       prediction.

       If you are intending to keep the file pointer open for use  in  libpng,
       you  must ensure you don't read more than 8 bytes from the beginning of
       the file, and you also have to make a call to png_set_sig_bytes()  with
       the number of bytes you read from the beginning.  Libpng will then only
       check the bytes (if any) that your program didn't read.

       (*): If you are not using the standard I/O functions, you will need  to
       replace them with custom functions.  See the discussion under Customiz-
       ing libpng.


           FILE *fp = fopen(file_name, "rb");
           if (!fp)
           {
               return (ERROR);
           }
           fread(header, 1, number, fp);
           is_png = !png_sig_cmp(header, 0, number);
           if (!is_png)
           {
               return (NOT_PNG);
           }


       Next, png_struct and png_info need to be allocated and initialized.  In
       order  to ensure that the size of these structures is correct even with
       a dynamically linked libpng, there  are  functions  to  initialize  and
       allocate  the  structures.   We also pass the library version, optional
       pointers to error handling functions, and a pointer to  a  data  struct
       for use by the error functions, if necessary (the pointer and functions
       can be NULL if the default error handlers are to  be  used).   See  the
       section  on  Changes  to  Libpng below regarding the old initialization
       functions.  The structure allocation functions quietly return  NULL  if
       they fail to create the structure, so your application should check for
       that.

           png_structp png_ptr = png_create_read_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);
           if (!png_ptr)
               return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
               png_destroy_read_struct(&png_ptr,
                  (png_infopp)NULL, (png_infopp)NULL);
               return (ERROR);
           }

           png_infop end_info = png_create_info_struct(png_ptr);
           if (!end_info)
           {
               png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
               return (ERROR);
           }

       If you  want  to  use  your  own  memory  allocation  routines,  define
       PNG_USER_MEM_SUPPORTED  and  use  png_create_read_struct_2() instead of
       png_create_read_struct():

           png_structp png_ptr = png_create_read_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       The error handling routines passed to png_create_read_struct() and  the
       memory  alloc/free  routines  passed  to png_create_struct_2() are only
       necessary if you are not using the libpng supplied error  handling  and
       memory alloc/free functions.

       When  libpng  encounters  an  error, it expects to longjmp back to your
       routine.  Therefore, you  will  need  to  call  setjmp  and  pass  your
       png_jmpbuf(png_ptr).  If you read the file from different routines, you
       will need to update the jmpbuf field every time you enter a new routine
       that will call a png_*() function.

       See  your  documentation  of  setjmp/longjmp for your compiler for more
       information on setjmp/longjmp.  See the discussion on libpng error han-
       dling  in  the Customizing Libpng section below for more information on
       the libpng error handling.  If an error occurs,  and  libpng  longjmp's
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
               png_destroy_read_struct(&png_ptr, &info_ptr,
                  &end_info);
               fclose(fp);
               return (ERROR);
           }

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now  you  need  to set up the input code.  The default for libpng is to
       use the C function fread().  If you use this, you will need to  pass  a
       valid  FILE  * in the function png_init_io().  Be sure that the file is
       opened in binary mode.  If you wish to handle reading data  in  another
       way,  you  need  not call the png_init_io() function, but you must then
       implement the libpng I/O methods discussed in  the  Customizing  Libpng
       section below.

           png_init_io(png_ptr, fp);

       If  you  had  previously  opened the file and read any of the signature
       from the beginning in order to see if this was a PNG file, you need  to
       let libpng know that there are some bytes missing from the start of the
       file.

           png_set_sig_bytes(png_ptr, number);

       You can change the zlib compression buffer size to be used while  read-
       ing compressed data with

           png_set_compression_buffer_size(png_ptr, buffer_size);

       where  the  default  size  is 8192 bytes.  Note that the buffer size is
       changed immediately and the buffer is reallocated immediately,  instead
       of setting a flag to be acted upon later.


   Setting up callback code
       You  can set up a callback function to handle any unknown chunks in the
       input stream. You must supply the function

           read_chunk_callback(png_structp png_ptr,
                png_unknown_chunkp chunk);
           {
              /* The unknown chunk structure contains your
                 chunk data, along with similar data for any other
                 unknown chunks: */

                  png_byte name[5];
                  png_byte *data;
                  png_size_t size;

              /* Note that libpng has already taken care of
                 the CRC handling */

              /* put your code here.  Search for your chunk in the
                 unknown chunk structure, process it, and return one
                 of the following: */

              return (-n); /* chunk had an error */
              return (0); /* did not recognize */
              return (n); /* success */
           }

       (You can give your function another  name  that  you  like  instead  of
       "read_chunk_callback")

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

       This names not only the callback function, but also a user pointer that
       you can retrieve with

           png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown
       chunks  will  be  saved  when read, in case your callback function will
       need one or more of them.   This  behavior  can  be  changed  with  the
       png_set_keep_unknown_chunks() function, described below.

       At  this  point, you can set up a callback function that will be called
       after each row has been read, which you can use to control  a  progress
       meter  or the like.  It's demonstrated in pngtest.c.  You must supply a
       function

           void read_row_callback(png_structp png_ptr,
              png_uint_32 row, int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of  "read_row_call-
       back")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);


   Unknown-chunk handling
       Now  you get to set the way the library processes unknown chunks in the
       input PNG stream. Both known and unknown chunks will be  read.   Normal
       behavior  is that known chunks will be parsed into information in vari-
       ous info_ptr members while  unknown  chunks  will  be  discarded.  This
       behavior  can be wasteful if your application will never use some known
       chunk types. To change this, you can call:

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);
           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy
                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3
           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is 0)
           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If nonzero,
                        only the chunks in the list are affected

       Unknown chunks declared in this way will be saved as raw  data  onto  a
       list  of  png_unknown_chunk  structures.   If  a chunk that is normally
       known to libpng is named in the list, it will be  handled  as  unknown,
       according  to  the "keep" directive.  If a chunk is named in successive
       instances of png_set_keep_unknown_chunks(),  the  final  instance  will
       take  precedence.   The  IHDR  and  IEND  chunks should not be named in
       chunk_list; if they are, libpng will process them normally anyway.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the  private "vpAg" chunk will later be processed by a user chunk call-
       back function:

           png_byte vpAg[5]={118, 112,  65, 103, (png_byte) ' '};

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             png_byte unused_chunks[]=
             {
               104,  73,  83,  84, (png_byte) ' ',   /* hIST */
               105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
               112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
               115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
               115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
               116,  73,  77,  69, (png_byte) ' ',   /* tIME */
             };
           #endif

           ...

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             /* ignore all unknown chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
             /* except for vpAg: */
             png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
             /* also ignore unused known chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
                (int)sizeof(unused_chunks)/5);
           #endif


   User limits
       The PNG specification allows the width and height of an image to be  as
       large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and columns.
       Since very few applications really need to process such  large  images,
       we  have  imposed  an  arbitrary  1-million  limit on rows and columns.
       Larger images will be rejected immediately with a png_error() call.  If
       you wish to override this limit, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to  set your own limits, or use width_max = height_max = 0x7fffffffL to
       allow all valid dimensions (libpng may reject some  very  large  images
       anyway because of potential buffer overflow conditions).

       You  should  put  this statement after you create the PNG structure and
       before calling png_read_info(), png_read_png(), or  png_process_data().
       If you need to retrieve the limits that are being applied, use

          width_max = png_get_user_width_max(png_ptr);
          height_max = png_get_user_height_max(png_ptr);

       The  PNG  specification sets no limit on the number of ancillary chunks
       allowed in a PNG datastream.  You can impose a limit on the total  num-
       ber  of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored,
       with

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

       where 0x7fffffffL means unlimited.  You can retrieve this limit with

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       This limit also applies to the number of buffers that can be  allocated
       by  png_decompress_chunk()  while  decompressing  iTXt,  zTXt, and iCCP
       chunks.

       You can also set a limit on the amount  of  memory  that  a  compressed
       chunk other than IDAT can occupy, with

          png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

          chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

       Any  chunks that would cause either of these limits to be exceeded will
       be ignored.


   The high-level read interface
       At this point there are two ways to  proceed;  through  the  high-level
       read  interface,  or  through  a sequence of low-level read operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input transformations you want to
       do are limited to the following set:

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_STRIP_16      Strip 16-bit samples to
                                       8 bits
           PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
           PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                       samples to bytes
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_EXPAND        Perform set_expand()
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                       to RGB (or GA to RGBA)

       (This excludes setting a background color, doing gamma  transformation,
       quantizing, and setting filler.)  If this is the case, simply do this:

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_read_info(),  followed  the set of transformations indicated by the
       transform mask, then png_read_image(), and finally png_read_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point to transformation parameters required by some future input trans-
       form.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_read_png().

       After  you  have called png_read_png(), you can retrieve the image data
       with

          row_pointers = png_get_rows(png_ptr, info_ptr);

       where row_pointers is an array of pointers to the pixel data  for  each
       row:

          png_bytep row_pointers[height];

       If you know your image size and pixel size ahead of time, you can allo-
       cate row_pointers prior to calling png_read_png() with

          if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
             png_error (png_ptr,
                "Image is too tall to process in memory");
          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                "Image is too wide to process in memory");
          row_pointers = png_malloc(png_ptr,
             height*png_sizeof(png_bytep));
          for (int i=0; i<height, i++)
             row_pointers[i]=NULL;  /* security precaution */
          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                width*pixel_size);
          png_set_rows(png_ptr, info_ptr, &row_pointers);

       Alternatively you could allocate your image in one big block and define
       row_pointers[i] to point into the proper places in your block.

       If  you  use png_set_rows(), the application is responsible for freeing
       row_pointers (and row_pointers[i], if they were separately allocated).

       If you don't allocate row_pointers ahead of time,  png_read_png()  will
       do it, and it'll be free'ed when you call png_destroy_*().


   The low-level read interface
       If you are going the low-level route, you are now ready to read all the
       file information up to the actual image data.  You do this with a  call
       to png_read_info().

           png_read_info(png_ptr, info_ptr);

       This will process all chunks up to but not including the image data.


   Querying the info structure
       Functions are used to get the information from the info_ptr once it has
       been read.  Note that these fields may  not  be  completely  filled  in
       until png_read_end() has read the chunk data following the image.

           png_get_IHDR(png_ptr, info_ptr, &width, &height,
              &bit_depth, &color_type, &interlace_type,
              &compression_type, &filter_method);

           width          - holds the width of the image
                            in pixels (up to 2^31).
           height         - holds the height of the image
                            in pixels (up to 2^31).
           bit_depth      - holds the bit depth of one of the
                            image channels.  (valid values are
                            1, 2, 4, 8, 16 and depend also on
                            the color_type.  See also
                            significant bits (sBIT) below).
           color_type     - describes which color/alpha channels
                                are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)

           Any or all of interlace_type, compression_type, or
           filter_method can be NULL if you are
           not interested in their values.

           Note that png_get_IHDR() returns 32-bit data into
           the application's width and height variables.
           This is an unsafe situation if these are 16-bit
           variables.  In such situations, the
           png_get_image_width() and png_get_image_height()
           functions described below are safer.

           width            = png_get_image_width(png_ptr,
                                info_ptr);
           height           = png_get_image_height(png_ptr,
                                info_ptr);
           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);
           color_type       = png_get_color_type(png_ptr,
                                info_ptr);
           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);
           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);
           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);
           channels       - number of channels of info for the
                            color type (valid values are 1 (GRAY,
                            PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                            4 (RGB_ALPHA or RGB + filler byte))
           rowbytes = png_get_rowbytes(png_ptr, info_ptr);
           rowbytes       - number of bytes needed to hold a row

           signature = png_get_signature(png_ptr, info_ptr);
           signature      - holds the signature read from the
                            file (if any).  The data is kept in
                            the same offset it would be if the
                            whole signature were read (i.e. if an
                            application had already read in 4
                            bytes of signature before starting
                            libpng, the remaining 4 bytes would
                            be in signature[4] through signature[7]
                            (see png_set_sig_bytes())).

       These  are  also  important,  but their validity depends on whether the
       chunk   has   been   read.    The   png_get_valid(png_ptr,    info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return non-zero if the data has been read, or zero if  it  is  missing.
       The parameters to the png_get_<chunk> are set directly if they are sim-
       ple data types, or a pointer into the info_ptr is returned for any com-
       plex types.

           png_get_PLTE(png_ptr, info_ptr, &palette,
                            &num_palette);
           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

           png_get_gAMA(png_ptr, info_ptr, &gamma);
           gamma          - the gamma the file is written
                            at (PNG_INFO_gAMA)

           png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
           srgb_intent    - the rendering intent (PNG_INFO_sRGB)
                            The presence of the sRGB chunk
                            means that the pixel data is in the
                            sRGB color space.  This chunk also
                            implies specific values of gAMA and
                            cHRM.

           png_get_iCCP(png_ptr, info_ptr, &name,
              &compression_type, &profile, &proflen);
           name             - The profile name.
           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.
           profile          - International Color Consortium color
                              profile data. May contain NULs.
           proflen          - length of profile data in bytes.

           png_get_sBIT(png_ptr, info_ptr, &sig_bit);
           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray,
                            red, green, and blue channels,
                            whichever are appropriate for the
                            given color type (png_color_16)

           png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                            &num_trans, &trans_color);
           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)
           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)
           trans_color    - graylevel or color sample values of
                            the single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_get_hIST(png_ptr, info_ptr, &hist);
                            (PNG_INFO_hIST)
           hist           - histogram of palette (array of
                            png_uint_16)

           png_get_tIME(png_ptr, info_ptr, &mod_time);
           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);
           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)
                            valid 16-bit red, green and blue
                            values, regardless of color_type

           num_comments   = png_get_text(png_ptr, info_ptr,
                            &text_ptr, &num_text);
           num_comments   - number of comments
           text_ptr       - array of png_text holding image
                            comments
           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                                1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (empty
                                string for unknown).
           text_ptr[i].lang_key  - keyword in UTF-8
                                (empty string for unknown).
           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist
           when the library is built with iTXt chunk support.

           num_text       - number of comments (same as
                            num_comments; you can put NULL here
                            to avoid the duplication)
           Note while png_set_text() will accept text, language,
           and translated keywords that can be NULL pointers, the
           structure returned by png_get_text will always contain
           regular zero-terminated C strings.  They might be
           empty strings but they will never be NULL pointers.

           num_spalettes = png_get_sPLT(png_ptr, info_ptr,
              &palette_ptr);
           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.
           num_spalettes  - number of sPLT chunks read.

           png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
              &unit_type);
           offset_x       - positive offset from the left edge
                            of the screen
           offset_y       - positive offset from the top edge
                            of the screen
           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
              &unit_type);
           res_x          - pixels/unit physical resolution in
                            x direction
           res_y          - pixels/unit physical resolution in
                            x direction
           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

           png_get_sCAL(png_ptr, info_ptr, &unit, &width,
              &height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are doubles)

           png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
              &height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)
           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position of chunk in file

           The value of "i" corresponds to the order in which the
           chunks were read from the PNG file or inserted with the
           png_set_unknown_chunks() function.

       The  data  from  the  pHYs chunk can be retrieved in several convenient
       forms:

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)
           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)
           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)
           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)
           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)
           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)
           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

          (Each of these returns 0 [signifying "unknown"] if
              the data is not present or if res_x is 0;
              res_x_and_y is 0 if res_x != res_y)

       The data from the oFFs chunk can be  retrieved  in  several  convenient
       forms:

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

          (Each of these returns 0 [signifying "unknown" if both
              x and y are 0] if the data is not present or if the
              chunk is present but the unit is the pixel)

       For  more information, see the png_info definition in png.h and the PNG
       specification for chunk contents.  Be careful with  trusting  rowbytes,
       as  some of the transformations could increase the space needed to hold
       a row (expand, filler, gray_to_rgb, etc.).  See png_read_update_info(),
       below.

       A  quick word about text_ptr and num_text.  PNG stores comments in key-
       word/text pairs, one pair per chunk, with no limit  on  the  number  of
       text chunks, and a 2^31 byte limit on their size.  While there are sug-
       gested keywords, there is no requirement to restrict the use  to  these
       strings.   It  is strongly suggested that keywords and text be sensible
       to humans (that's the point), so don't use abbreviations.  Non-printing
       symbols  are  not allowed.  See the PNG specification for more details.
       There is also no requirement to have text after the keyword.

       Keywords should be limited to 79 Latin-1 characters without leading  or
       trailing spaces, but non-consecutive spaces are allowed within the key-
       word.  It is possible to have the same keyword  any  number  of  times.
       The text_ptr is an array of png_text structures, each holding a pointer
       to a language string, a pointer to a keyword and a pointer  to  a  text
       string.   The text string, language code, and translated keyword may be
       empty or NULL pointers.  The keyword/text pairs are put into the  array
       in  the order that they are received.  However, some or all of the text
       chunks may be after the image, so, to make sure you have read  all  the
       text chunks, don't mess with these until after you read the stuff after
       the image.  This will be mentioned again below in the  discussion  that
       goes with png_read_end().


   Input transformations
       After you've read the header information, you can set up the library to
       handle any special transformations of the image data.  The various ways
       to  transform  the data will be described in the order that they should
       occur.  This is important, as some  of  these  change  the  color  type
       and/or  bit  depth  of  the  data, and some others only work on certain
       color types and bit depths.  Even though each transformation checks  to
       see  if it has data that it can do something with, you should make sure
       to only enable a transformation if it will be valid for the data.   For
       example, don't swap red and blue on grayscale data.

       The  colors  used  for the background and transparency values should be
       supplied in the same format/depth as the current image data.  They  are
       stored  in  the  same  format/depth as the image data in a bKGD or tRNS
       chunk, so this is what libpng expects for this data.   The  colors  are
       transformed  to  keep  in  sync with the image data when an application
       calls the png_read_update_info() routine (see below).

       Data will be decoded into the supplied row buffers  packed  into  bytes
       unless  the  library has been told to transform it into another format.
       For example, 4 bit/pixel paletted or grayscale data will be returned  2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless png_set_packing() is called.  8-bit RGB data will be  stored  in
       RGB  RGB  RGB  format unless png_set_filler() or png_set_add_alpha() is
       called to insert filler bytes, either before or after each RGB triplet.
       16-bit  RGB data will be returned RRGGBB RRGGBB, with the most signifi-
       cant byte of the color value first, unless png_set_strip_16() is called
       to  transform  it  to  regular RGB RGB triplets, or png_set_filler() or
       png_set_add alpha() is called to insert filler bytes, either before  or
       after  each  RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data
       can  be  modified  with   png_set_filler(),   png_set_add_alpha(),   or
       png_set_strip_16().

       The  following  code  transforms  grayscale  images of less than 8 to 8
       bits, changes paletted images to RGB, and adds a full alpha channel  if
       there is transparency information in a tRNS chunk.  This is most useful
       on grayscale images with bit depths of 2 or 4 or if there is  a  multi-
       ple-image  viewing  application  that wishes to treat all images in the
       same way.

           if (color_type == PNG_COLOR_TYPE_PALETTE)
               png_set_palette_to_rgb(png_ptr);

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

           if (png_get_valid(png_ptr, info_ptr,
               PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

       These three functions are actually aliases for png_set_expand(),  added
       in  libpng  version  1.0.4, with the function names expanded to improve
       code readability.  In some future version they may actually do  differ-
       ent things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As of libpng version 1.4.20, not all possible expansions are supported.

       In the following table, the 01 means grayscale with depth<8,  31  means
       indexed  with  depth<8,  other  numerals  represent the color type, "T"
       means the tRNS chunk is present, A means an alpha channel  is  present,
       and  O  means  tRNS or alpha is present but all pixels in the image are
       opaque.

         FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
          TO
          01    -
          31        -
           0    1       -
          0T                -
          0O                    -
           2           GX           -
          2T                            -
          2O                                -
           3        1                           -
          3T                                        -
          3O                                            -
          4A                T                               -
          4O                                                    -
          6A               GX         TX           TX               -
          6O                   GX                      TX               -

       Within the matrix,
            "-" means the transformation is not supported.
            "X" means the transformation is obtained by png_set_expand().
            "1" means the transformation is obtained by
                png_set_expand_gray_1_2_4_to_8
            "G" means the transformation is obtained by
                png_set_gray_to_rgb().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().

       PNG can have files with 16 bits per channel.  If you only can handle  8
       bits per channel, this will strip the pixels down to 8 bit.

           if (bit_depth == 16)
               png_set_strip_16(png_ptr);

       If,  for some reason, you don't need the alpha channel on an image, and
       you want to remove it rather than combining it with the background (but
       the image author certainly had in mind that you *would* combine it with
       the background, so that's what you should probably do):

           if (color_type & PNG_COLOR_MASK_ALPHA)
               png_set_strip_alpha(png_ptr);

       In PNG files, the alpha channel in an image is the  level  of  opacity.
       If  you  need  the  alpha channel in an image to be the level of trans-
       parency instead of opacity, you can invert the alpha  channel  (or  the
       tRNS chunk data) after it's read, so that 0 is fully opaque and 255 (in
       8-bit or paletted images) or 65535 (in 16-bit images) is  fully  trans-
       parent, with

           png_set_invert_alpha(png_ptr);

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       This  code  expands  to 1 pixel per byte without changing the values of
       the pixels:

           if (bit_depth < 8)
               png_set_packing(png_ptr);

       PNG files have possible bit depths of 1, 2, 4, 8, and 16.   All  pixels
       stored  in  a  PNG image have been "scaled" or "shifted" up to the next
       higher possible bit depth (e.g. from 5 bits/sample in the range  [0,31]
       to  8 bits/sample in the range [0, 255]).  However, it is also possible
       to convert the PNG pixel data back to the original  bit  depth  of  the
       image.   This  call  reduces  the  pixels back down to the original bit
       depth:

           png_color_8p sig_bit;

           if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
               png_set_shift(png_ptr, sig_bit);

       PNG files store 3-color pixels in red, green, blue  order.   This  code
       changes the storage of the pixels to blue, green, red:

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
               png_set_bgr(png_ptr);

       PNG  files store RGB pixels packed into 3 or 6 bytes. This code expands
       them into 4 or 8 bytes for windowing systems that  need  them  in  this
       format:

           if (color_type == PNG_COLOR_TYPE_RGB)
               png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

       where "filler" is the 8 or 16-bit number to fill with, and the location
       is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
       you  want the filler before the RGB or after.  This transformation does
       not affect images that already have full alpha  channels.   To  add  an
       opaque  alpha  channel,  use filler=0xff or 0xffff and PNG_FILLER_AFTER
       which will generate RGBA pixels.

       Note that png_set_filler() does not change the color type.  If you want
       to do that, you can add a true alpha channel with

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_GRAY)
           png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

       where  "filler" contains the alpha value to assign to each pixel.  This
       function was added in libpng-1.2.7.

       If you are reading an image with an alpha channel,  and  you  need  the
       data as ARGB instead of the normal PNG format RGBA:

           if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
               png_set_swap_alpha(png_ptr);

       For some uses, you may want a grayscale image to be represented as RGB.
       This code will do that conversion:

           if (color_type == PNG_COLOR_TYPE_GRAY ||
               color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
                 png_set_gray_to_rgb(png_ptr);

       Conversely, you can convert an  RGB  or  RGBA  image  to  grayscale  or
       grayscale with alpha.

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
                 png_set_rgb_to_gray_fixed(png_ptr, error_action,
                    int red_weight, int green_weight);

           error_action = 1: silently do the conversion
           error_action = 2: issue a warning if the original
                             image has any pixel where
                             red != green or red != blue
           error_action = 3: issue an error and abort the
                             conversion if the original
                             image has any pixel where
                             red != green or red != blue

           red_weight:       weight of red component times 100000
           green_weight:     weight of green component times 100000
                             If either weight is negative, default
                             weights (21268, 71514) are used.

       If  you have set error_action = 1 or 2, you can later check whether the
       image really was gray,  after  processing  the  image  rows,  with  the
       png_get_rgb_to_gray_status(png_ptr)   function.    It   will  return  a
       png_byte that is zero if the image was gray or 1 if there were any non-
       gray  pixels.   bKGD  and  sBIT  data  will  be  silently  converted to
       grayscale, using the green channel data, regardless of the error_action
       setting.

       With  red_weight+green_weight<=100000, the normalized graylevel is com-
       puted:

           int rw = red_weight * 65536;
           int gw = green_weight * 65536;
           int bw = 65536 - (rw + gw);
           gray = (rw*red + gw*green + bw*blue)/65536;

       The defaults correspond to the ITU-R recommendation 709, and  also  the
       sRGB  color  space, as recommended in the Charles Poynton's Colour FAQ,
       Copyright (c) 2006-11-28 Charles Poynton, in section 9:

       <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

           Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

       Libpng approximates this with

           Y = 0.21268 * R    + 0.7151 * G    + 0.07217 * B

       which can be expressed with integers as

           Y = (6969 * R + 23434 * G + 2365 * B)/32768

       The calculation is done in a linear colorspace, if the image  gamma  is
       known.

       If  you  have  a  grayscale  and  you are using png_set_expand_depth(),
       png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to a
       higher bit-depth, you must either supply the background color as a gray
       value at the original file bit-depth (need_expand = 1) or  else  supply
       the background color as an RGB triplet at the final, expanded bit depth
       (need_expand = 0).  Similarly, if you are reading a paletted image, you
       must either supply the background color as a palette index (need_expand
       = 1) or as an RGB triplet that  may  or  may  not  be  in  the  palette
       (need_expand = 0).

           png_color_16 my_background;
           png_color_16p image_background;

           if (png_get_bKGD(png_ptr, info_ptr, &image_background))
               png_set_background(png_ptr, image_background,
                 PNG_BACKGROUND_GAMMA_FILE, 1, 1.0);
           else
               png_set_background(png_ptr, &my_background,
                 PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);

       The png_set_background() function tells libpng to composite images with
       alpha or simple transparency against the supplied background color.  If
       the  PNG  file contains a bKGD chunk (PNG_INFO_bKGD valid), you may use
       this color, or supply another color more suitable for the current  dis-
       play  (e.g.,  the  background color from a web page).  You need to tell
       libpng whether  the  color  is  in  the  gamma  space  of  the  display
       (PNG_BACKGROUND_GAMMA_SCREEN   for   colors   you   supply),  the  file
       (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one that
       is  neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't know
       why anyone would use this, but it's here).

       To properly display PNG images on any kind of system,  the  application
       needs  to  know what the display gamma is.  Ideally, the user will know
       this, and the application will allow them to set  it.   One  method  of
       allowing  the  user to set the display gamma separately for each system
       is to check for a SCREEN_GAMMA or DISPLAY_GAMMA  environment  variable,
       which will hopefully be correctly set.

       Note  that  display_gamma  is  the overall gamma correction required to
       produce pleasing results, which depends on the lighting  conditions  in
       the surrounding environment.  In a dim or brightly lit room, no compen-
       sation other than the physical gamma exponent of the monitor is needed,
       while in a dark room a slightly smaller exponent is better.

          double gamma, screen_gamma;

          if (/* We have a user-defined screen
              gamma value */)
          {
             screen_gamma = user_defined_screen_gamma;
          }
          /* One way that applications can share the same
             screen gamma value */
          else if ((gamma_str = getenv("SCREEN_GAMMA"))
             != NULL)
          {
             screen_gamma = (double)atof(gamma_str);
          }
          /* If we don't have another value */
          else
          {
             screen_gamma = 2.2; /* A good guess for a
                  PC monitor in a bright office or a dim room */
             screen_gamma = 2.0; /* A good guess for a
                  PC monitor in a dark room */
             screen_gamma = 1.7 or 1.0;  /* A good
                  guess for Mac systems */
          }

       The png_set_gamma() function handles gamma transformations of the data.
       Pass both the file gamma and the current  screen_gamma.   If  the  file
       does  not  have  a  gamma value, you can pass one anyway if you have an
       idea what it is (usually 0.45455 is a good  guess  for  GIF  images  on
       PCs).   Note that file gammas are inverted from screen gammas.  See the
       discussions on gamma in the PNG specification for an excellent descrip-
       tion  of what gamma is, and why all applications should support it.  It
       is strongly recommended that PNG viewers support gamma correction.

          if (png_get_gAMA(png_ptr, info_ptr, &gamma))
             png_set_gamma(png_ptr, screen_gamma, gamma);
          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If you need to reduce an RGB file to a paletted file, or if a  paletted
       file  has more entries then will fit on your screen, png_set_quantize()
       will do that.  Note that this is a  simple  match  dither  that  merely
       finds  the  closest color available.  This should work fairly well with
       optimized palettes, and fairly badly with linear color cubes.   If  you
       pass a palette that is larger then maximum_colors, the file will reduce
       the number of colors in the palette so it will fit into maximum_colors.
       If  there  is  a  histogram,  it  will  use it to make more intelligent
       choices when reducing the palette.  If there is no  histogram,  it  may
       not do as good a job.

          if (color_type & PNG_COLOR_MASK_COLOR)
          {
             if (png_get_valid(png_ptr, info_ptr,
                PNG_INFO_PLTE))
             {
                png_uint_16p histogram = NULL;
                png_get_hIST(png_ptr, info_ptr,
                   &histogram);
                png_set_quantize(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }
             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

                png_set_quantize(png_ptr, std_color_cube,
                   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                   NULL,0);
             }
          }

       PNG  files describe monochrome as black being zero and white being one.
       The following code will reverse this (make black be one  and  white  be
       zero):

          if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
             png_set_invert_mono(png_ptr);

       This  function  can  also  be  used  to invert grayscale and gray-alpha
       images:

          if (color_type == PNG_COLOR_TYPE_GRAY ||
               color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
             png_set_invert_mono(png_ptr);

       PNG files store 16 bit pixels in network byte  order  (big-endian,  ie.
       most  significant  bits  first).   This code changes the storage to the
       other way (little-endian, i.e. least significant bits  first,  the  way
       PCs store them):

           if (bit_depth == 16)
               png_set_swap(png_ptr);

       If  you  are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_read_user_transform_fn(png_ptr,
              read_transform_fn);

       You must supply the function

           void read_transform_fn(png_structp png_ptr, png_row_infop
               row_info, png_bytep data)

       See pngtest.c for a working example.   Your  function  will  be  called
       after all of the other transformations have been processed.

       You can also set up a pointer to a user structure for use by your call-
       back function, and you can inform libpng that your  transform  function
       will change the number of channels or bit depth with the function

           png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

       The  user's  application, not libpng, is responsible for allocating and
       freeing any memory required for the user structure.

       You can retrieve  the  pointer  via  the  function  png_get_user_trans-
       form_ptr().  For example:

           voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       The  last  thing  to  handle  is interlacing; this is covered in detail
       below, but you must call the function here if you want libpng to handle
       expansion of the interlaced image.

           number_of_passes = png_set_interlace_handling(png_ptr);

       After  setting  the  transformations,  libpng  can update your png_info
       structure to reflect any transformations  you've  requested  with  this
       call.   This  is  most  useful  to update the info structure's rowbytes
       field so you can use it to allocate your image memory.   This  function
       will  also  update your palette with the correct screen_gamma and back-
       ground if these have been given with the calls above.

           png_read_update_info(png_ptr, info_ptr);

       After you call png_read_update_info(), you can allocate any memory  you
       need  to  hold the image.  The row data is simply raw byte data for all
       forms of images.  As the actual allocation varies  among  applications,
       no  example  will be given.  If you are allocating one large chunk, you
       will need to build an array of pointers to each  row,  as  it  will  be
       needed for some of the functions below.


   Reading image data
       After  you've  allocated memory, you can read the image data.  The sim-
       plest way to do this is in one function call.  If  you  are  allocating
       enough   memory   to   hold   the   whole  image,  you  can  just  call
       png_read_image() and libpng will read in all the image data and put  it
       in  the  memory  area  supplied.   You will need to pass in an array of
       pointers to each row.

       This function automatically handles interlacing, so you don't  need  to
       call png_set_interlace_handling() or call this function multiple times,
       or any of that other stuff necessary with png_read_rows().

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If you don't want to read in the whole  image  at  once,  you  can  use
       png_read_rows()  instead.   If  there  is  no interlacing (check inter-
       lace_type == PNG_INTERLACE_NONE), this is simple:

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

       where row_pointers is the same as in the png_read_image() call.

       If you are doing this just one row at a time, you can do  this  with  a
       single row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;
           png_read_row(png_ptr, row_pointer, NULL);

       If  the  file  is  interlaced  (interlace_type != 0 in the IHDR chunk),
       things get somewhat harder.  The only current (PNG  Specification  ver-
       sion  1.2)  interlacing  type  for PNG is (interlace_type == PNG_INTER-
       LACE_ADAM7) is a somewhat complicated 2D  interlace  scheme,  known  as
       Adam7,  that  breaks down an image into seven smaller images of varying
       size, based on an 8x8 grid.

       libpng can fill out those images or it can give them to  you  "as  is".
       If  you  want  them filled out, there are two ways to do that.  The one
       mentioned in the PNG specification is to expand  each  pixel  to  cover
       those  pixels  that  have  not  been read yet (the "rectangle" method).
       This results in a blocky image for  the  first  pass,  which  gradually
       smooths out as more pixels are read.  The other method is the "sparkle"
       method, where pixels are drawn only in their final locations, with  the
       rest  of  the  image remaining whatever colors they were initialized to
       before the start of the read.  The first method usually  looks  better,
       but tends to be slower, as there are more pixels to put in the rows.

       If  you  don't want libpng to handle the interlacing details, just call
       png_read_rows() seven times to read in all seven images.  Each  of  the
       images  is  a  valid image by itself, or they can all be combined on an
       8x8 grid to form a single image (although if you intend to combine them
       you would be far better off using the libpng interlace handling).

       The  first  pass  will  return an image 1/8 as wide as the entire image
       (every 8th column starting in column 0) and 1/8 as high as the original
       (every  8th  row  starting  in  row  0), the second will be 1/8 as wide
       (starting in column 4) and 1/8 as high (also starting in row  0).   The
       third  pass  will be 1/4 as wide (every 4th pixel starting in column 0)
       and 1/8 as high (every 8th row starting in row 4), and the fourth  pass
       will  be 1/4 as wide and 1/4 as high (every 4th column starting in col-
       umn 2, and every 4th row starting in  row  0).   The  fifth  pass  will
       return  an image 1/2 as wide, and 1/4 as high (starting at column 0 and
       row 2), while the sixth pass will be 1/2 as wide and 1/2 as high as the
       original  (starting in column 1 and row 0).  The seventh and final pass
       will be as wide as the original, and 1/2 as high, containing all of the
       odd numbered scanlines.  Phew!

       If  you  want  libpng  to  expand  the images, call this before calling
       png_start_read_image() or png_read_update_info():

           if (interlace_type == PNG_INTERLACE_ADAM7)
               number_of_passes
                  = png_set_interlace_handling(png_ptr);

       This will return the number  of  passes  needed.   Currently,  this  is
       seven,  but  may change if another interlace type is added.  This func-
       tion can be called even if the file is not interlaced,  where  it  will
       return one pass.

       If  you  are  not  going  to display the image after each pass, but are
       going to wait until the entire  image  is  read  in,  use  the  sparkle
       effect.   This  effect is faster and the end result of either method is
       exactly the same.  If you are planning on displaying  the  image  after
       each  pass,  the  "rectangle" effect is generally considered the better
       looking one.

       If you only want the "sparkle" effect,  just  call  png_read_rows()  as
       normal,  with  the  third parameter NULL.  Make sure you make pass over
       the image number_of_passes times, and you don't change the data in  the
       rows between calls.  You can change the locations of the data, just not
       the data.  Each pass only writes the pixels appropriate for that  pass,
       and assumes the data from previous passes is still valid.

           png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

       If  you  only  want  the  first effect (the rectangles), do the same as
       before except pass the row buffer in the third parameter, and leave the
       second parameter NULL.

           png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);


   Finishing a sequential read
       After  you  are finished reading the image through the low-level inter-
       face, you can finish reading the file.  If you are interested  in  com-
       ments  or  time,  which  may be stored either before or after the image
       data, you should pass the separate png_info struct if you want to  keep
       the  comments from before and after the image separate.  If you are not
       interested, you can pass NULL.

          png_read_end(png_ptr, end_info);

       When you are done, you can free all memory  allocated  by  libpng  like
       this:

          png_destroy_read_struct(&png_ptr, &info_ptr,
              &end_info);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)
           mask - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL
           seq  - sequence number of item to be freed
                  (-1 for all items)

       This function may be  safely  called  when  the  relevant  storage  has
       already  been freed, or has not yet been allocated, or was allocated by
       the user and not by libpng,  and will in those cases do  nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was  allocated  by  the  user  with
       png_malloc() and passed in via a png_set_*() function, with

           png_data_freer(png_ptr, info_ptr, freer, mask)
           mask   - which data elements are affected
                    same choices as in png_free_data()
           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

       This  function  only affects data that has already been allocated.  You
       can call this function after reading the PNG data  but  before  calling
       any   png_set_*()  functions,  to  control  whether  the  user  or  the
       png_set_*() function is responsible for freeing any existing data  that
       might  be present, and again after the png_set_*() functions to control
       whether the user or png_destroy_*() is supposed to free the data.  When
       the user assumes responsibility for libpng-allocated data, the applica-
       tion must use png_free() to  free  it,  and  when  the  user  transfers
       responsibility to libpng for data that the user has allocated, the user
       must have used png_malloc() to allocate it.

       If you allocated your row_pointers in  a  single  block,  as  suggested
       above in the description of the high level read interface, you must not
       transfer  responsibility  for  freeing  it  to  the   png_set_rows   or
       png_read_destroy  function,  because  they  would  also try to free the
       individual row_pointers[i].

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans-
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text  structure  it
       combines  these  members  with the key member, and png_free_data() will
       free only text_ptr.key.  Similarly, if you transfer responsibility  for
       free'ing  text_ptr  from  libpng  to your application, your application
       must not separately free those members.

       The png_free_data() function will turn off the "valid"  flag  for  any-
       thing  it frees.  If you need to turn the flag off for a chunk that was
       freed by your application instead of by libpng, you can use

           png_set_invalid(png_ptr, info_ptr, mask);
           mask - identifies the chunks to be made invalid,
                  containing the bitwise OR of one or
                  more of
                    PNG_INFO_gAMA, PNG_INFO_sBIT,
                    PNG_INFO_cHRM, PNG_INFO_PLTE,
                    PNG_INFO_tRNS, PNG_INFO_bKGD,
                    PNG_INFO_hIST, PNG_INFO_pHYs,
                    PNG_INFO_oFFs, PNG_INFO_tIME,
                    PNG_INFO_pCAL, PNG_INFO_sRGB,
                    PNG_INFO_iCCP, PNG_INFO_sPLT,
                    PNG_INFO_sCAL, PNG_INFO_IDAT

       For a more compact example of reading a PNG image, see the  file  exam-
       ple.c.


   Reading PNG files progressively
       The  progressive  reader is slightly different then the non-progressive
       reader.   Instead  of  calling  png_read_info(),  png_read_rows(),  and
       png_read_end(),  you  make  one call to png_process_data(), which calls
       callbacks when it has the info, a row, or the end of  the  image.   You
       set  up  these callbacks with png_set_progressive_read_fn().  You don't
       have to worry about the input/output functions of libpng,  as  you  are
       giving  the  library  the  data directly in png_process_data().  I will
       assume that you have read the section on reading PNG files above, so  I
       will  only  highlight  the differences (although I will show all of the
       code).

       png_structp png_ptr; png_infop info_ptr;

        /*  An example code fragment of how you would
            initialize the progressive reader in your
            application. */
        int
        initialize_png_reader()
        {
           png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
                user_error_fn, user_warning_fn);
           if (!png_ptr)
               return (ERROR);
           info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
               png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
                  (png_infopp)NULL);
               return (ERROR);
           }

           if (setjmp(png_jmpbuf(png_ptr)))
           {
               png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
               return (ERROR);
           }

           /* This one's new.  You can provide functions
              to be called when the header info is valid,
              when each row is completed, and when the image
              is finished.  If you aren't using all functions,
              you can specify NULL parameters.  Even when all
              three functions are NULL, you need to call
              png_set_progressive_read_fn().  You can use
              any struct as the user_ptr (cast to a void pointer
              for the function call), and retrieve the pointer
              from inside the callbacks using the function

                 png_get_progressive_ptr(png_ptr);

              which will return a void pointer, which you have
              to cast appropriately.
            */
           png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
               info_callback, row_callback, end_callback);

           return 0;
        }

        /* A code fragment that you call as you receive blocks
          of data */
        int
        process_data(png_bytep buffer, png_uint_32 length)
        {
           if (setjmp(png_jmpbuf(png_ptr)))
           {
               png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
               return (ERROR);
           }

           /* This one's new also.  Simply give it a chunk
              of data from the file stream (in order, of
              course).  On machines with segmented memory
              models machines, don't give it any more than
              64K.  The library seems to run fine with sizes
              of 4K. Although you can give it much less if
              necessary (I assume you can give it chunks of
              1 byte, I haven't tried less then 256 bytes
              yet).  When this function returns, you may
              want to display any rows that were generated
              in the row callback if you don't already do
              so there.
            */
           png_process_data(png_ptr, info_ptr, buffer, length);
           return 0;
        }

        /* This function is called (as set by
           png_set_progressive_read_fn() above) when enough data
           has been supplied so all of the header has been
           read.
        */
        void
        info_callback(png_structp png_ptr, png_infop info)
        {
           /* Do any setup here, including setting any of
              the transformations mentioned in the Reading
              PNG files section.  For now, you _must_ call
              either png_start_read_image() or
              png_read_update_info() after all the
              transformations are set (even if you don't set
              any).  You may start getting rows before
              png_process_data() returns, so this is your
              last chance to prepare for that.
            */
        }

        /* This function is called when each row of image
           data is complete */
        void
        row_callback(png_structp png_ptr, png_bytep new_row,
           png_uint_32 row_num, int pass)
        {
           /* If the image is interlaced, and you turned
              on the interlace handler, this function will
              be called for every row in every pass.  Some
              of these rows will not be changed from the
              previous pass.  When the row is not changed,
              the new_row variable will be NULL.  The rows
              and passes are called in order, so you don't
              really need the row_num and pass, but I'm
              supplying them because it may make your life
              easier.

              For the non-NULL rows of interlaced images,
              you must call png_progressive_combine_row()
              passing in the row and the old row.  You can
              call this function for NULL rows (it will just
              return) and for non-interlaced images (it just
              does the memcpy for you) if it will make the
              code easier.  Thus, you can just do this for
              all cases:
            */

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

           /* where old_row is what was displayed for
              previously for the row.  Note that the first
              pass (pass == 0, really) will completely cover
              the old row, so the rows do not have to be
              initialized.  After the first pass (and only
              for interlaced images), you will have to pass
              the current row, and the function will combine
              the old row and the new row.
           */
        }

        void
        end_callback(png_structp png_ptr, png_infop info)
        {
           /* This function is called after the whole image
              has been read, including any chunks after the
              image (up to and including the IEND).  You
              will usually have the same info chunk as you
              had in the header, although some data may have
              been added to the comments and time fields.

              Most people won't do much here, perhaps setting
              a flag that marks the image as finished.
            */
        }




IV. Writing
       Much of this is very similar to reading.  However, everything of impor-
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.


   Setup
       You will want to do the I/O initialization before you get into  libpng,
       so  if it doesn't work, you don't have anything to undo. If you are not
       using the standard I/O functions, you will need to  replace  them  with
       custom writing functions.  See the discussion under Customizing libpng.

           FILE *fp = fopen(file_name, "wb");
           if (!fp)
           {
              return (ERROR);
           }

       Next, png_struct and png_info need to be allocated and initialized.  As
       these can be both relatively large, you may not want to store these  on
       the  stack,  unless you have stack space to spare.  Of course, you will
       want to check if they return NULL.  If you are also reading, you  won't
       want  to  name  your  read  structure  and  your  write  structure both
       "png_ptr"; you can call them anything you like, such as "read_ptr"  and
       "write_ptr".  Look at pngtest.c, for example.

           png_structp png_ptr = png_create_write_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);
           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
              png_destroy_write_struct(&png_ptr,
                (png_infopp)NULL);
              return (ERROR);
           }

       If  you  want  to  use  your  own  memory  allocation  routines, define
       PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2()  instead  of
       png_create_write_struct():

           png_structp png_ptr = png_create_write_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       After you have these structures, you will need to set up the error han-
       dling.  When libpng encounters an error, it expects to  longjmp()  back
       to  your  routine.   Therefore, you will need to call setjmp() and pass
       the png_jmpbuf(png_ptr).  If you write the  file  from  different  rou-
       tines,  you  will need to update the png_jmpbuf(png_ptr) every time you
       enter a new routine that will call a png_*() function.  See your  docu-
       mentation  of  setjmp/longjmp for your compiler for more information on
       setjmp/longjmp.  See the discussion on libpng  error  handling  in  the
       Customizing  Libpng  section  below  for more information on the libpng
       error handling.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_write_struct(&png_ptr, &info_ptr);
              fclose(fp);
              return (ERROR);
           }
           ...
           return;

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now  you  need to set up the output code.  The default for libpng is to
       use the C function fwrite().  If you use this, you will need to pass  a
       valid  FILE  * in the function png_init_io().  Be sure that the file is
       opened in binary mode.  Again, if you wish to handle  writing  data  in
       another way, see the discussion on libpng I/O handling in the Customiz-
       ing Libpng section below.

           png_init_io(png_ptr, fp);

       If you are embedding your PNG into a datastream such as MNG, and  don't
       want libpng to write the 8-byte signature, or if you have already writ-
       ten the signature in your application, use

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.


   Write callbacks
       At this point, you can set up a callback function that will  be  called
       after  each  row  has  been  written,  which  you  can use to control a
       progress meter or the like.  It's demonstrated in pngtest.c.  You  must
       supply a function

           void write_row_callback(png_structp png_ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "write_row_call-
       back")

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       You now have the option of modifying how the compression  library  will
       run.  The following functions are mainly for testing, but may be useful
       in some cases, like if you need to write PNG files extremely  fast  and
       are willing to give up some compression, or if you want to get the max-
       imum possible compression at the expense of  slower  writing.   If  you
       have no special needs in this area, let the library do what it wants by
       not calling this function at all, as it has been  tuned  to  deliver  a
       good  speed/compression ratio. The second parameter to png_set_filter()
       is the filter method, for which the only valid values are 0 (as of  the
       July  1999  PNG specification, version 1.2) or 64 (if you are writing a
       PNG datastream that is to be embedded in a MNG datastream).  The  third
       parameter  is  a  flag  that  indicates  which filter type(s) are to be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.


           /* turn on or off filtering, and/or choose
              specific filters.  You can use either a single
              PNG_FILTER_VALUE_NAME or the bitwise OR of one
              or more PNG_FILTER_NAME masks. */
           png_set_filter(png_ptr, 0,
              PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
              PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
              PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
              PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
              PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
              PNG_ALL_FILTERS);

       If an application wants to start and stop using particular filters dur-
       ing compression, it should start out with all of the filters (to ensure
       that  the  previous  row  of  pixels will be stored in case it's needed
       later), and then add and remove them after the start of compression.

       If you are writing a PNG datastream that is to be  embedded  in  a  MNG
       datastream, the second parameter can be either 0 or 64.

       The png_set_compression_*() functions interface to the zlib compression
       library, and should mostly be ignored unless you really know  what  you
       are   doing.   The  only  generally  useful  call  is  png_set_compres-
       sion_level() which changes how much time zlib spends on trying to  com-
       press  the  image  data.  See the Compression Library (zlib.h and algo-
       rithm.txt, distributed with zlib) for details on the  compression  lev-
       els.

           /* set the zlib compression level */
           png_set_compression_level(png_ptr,
               Z_BEST_COMPRESSION);

           /* set other zlib parameters */
           png_set_compression_mem_level(png_ptr, 8);
           png_set_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_compression_window_bits(png_ptr, 15);
           png_set_compression_method(png_ptr, 8);
           png_set_compression_buffer_size(png_ptr, 8192)

       extern PNG_EXPORT(void,png_set_zbuf_size)


   Setting the contents of info for output
       You  now  need  to fill in the png_info structure with all the data you
       wish to write before the actual image.  Note that the  only  thing  you
       are  allowed  to  write after the image is the text chunks and the time
       chunk (as of PNG Specification 1.2, anyway).  See  png_write_end()  and
       the latest PNG specification for more information on that.  If you wish
       to write them before the image, fill them in now, and flag that data as
       being valid.  If you want to wait until after the data, don't fill them
       until png_write_end().  For all the fields in png_info and  their  data
       types, see png.h.  For explanations of what the fields contain, see the
       PNG specification.

       Some of the more important parts of the png_info are:

           png_set_IHDR(png_ptr, info_ptr, width, height,
              bit_depth, color_type, interlace_type,
              compression_type, filter_method)
           width          - holds the width of the image
                            in pixels (up to 2^31).
           height         - holds the height of the image
                            in pixels (up to 2^31).
           bit_depth      - holds the bit depth of one of the
                            image channels.
                            (valid values are 1, 2, 4, 8, 16
                            and depend also on the
                            color_type.  See also significant
                            bits (sBIT) below).
           color_type     - describes which color/alpha
                            channels are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7
           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)
           filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                            or, if you are writing a PNG to
                            be embedded in a MNG datastream,
                            can also be
                            PNG_INTRAPIXEL_DIFFERENCING)

       If you call png_set_IHDR(), the call must  appear  before  any  of  the
       other  png_set_*() functions, because they might require access to some
       of the IHDR settings.

       If you wish, you can reset  the  compression_type,  interlace_type,  or
       filter_method  later  by  calling png_set_IHDR() again; if you do this,
       the width, height, bit_depth, and color_type must be the same  in  each
       call.

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);
           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

       If  you  call  png_set_PLTE(),  the  call  must appear before either of
       png_set_tRNS() or png_set_hIST() appears, because they  require  access
       to the palette length.

       The remaining png_set_*() functions can be called in any order.

           png_set_gAMA(png_ptr, info_ptr, gamma);
           gamma          - the gamma the image was created
                            at (PNG_INFO_gAMA)

           png_set_sRGB(png_ptr, info_ptr, srgb_intent);
           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of
                            the sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This chunk also implies specific
                            values of gAMA and cHRM.  Rendering
                            intent is the CSS-1 property that
                            has been defined by the International
                            Color Consortium
                            (http://www.color.org).
                            It can be one of
                            PNG_sRGB_INTENT_SATURATION,
                            PNG_sRGB_INTENT_PERCEPTUAL,
                            PNG_sRGB_INTENT_ABSOLUTE, or
                            PNG_sRGB_INTENT_RELATIVE.


           png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
              srgb_intent);
           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of the
                            sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This function also causes gAMA and
                            cHRM chunks with the specific values
                            that are consistent with sRGB to be
                            written.

           png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                             profile, proflen);
           name             - The profile name.
           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.
           profile          - International Color Consortium color
                              profile data. May contain NULs.
           proflen          - length of profile data in bytes.

           png_set_sBIT(png_ptr, info_ptr, sig_bit);
           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray, red,
                            green, and blue channels, whichever are
                            appropriate for the given color type
                            (png_color_16)

           png_set_tRNS(png_ptr, info_ptr, trans_alpha,
              num_trans, trans_color);
           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)
           trans_color    - graylevel or color sample values
                            (in order red, green, blue) of the
                            single transparent color for
                            non-paletted images (PNG_INFO_tRNS)
           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           png_set_hIST(png_ptr, info_ptr, hist);
                           (PNG_INFO_hIST)
           hist           - histogram of palette (array of
                            png_uint_16)

           png_set_tIME(png_ptr, info_ptr, mod_time);
           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);
           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);
           text_ptr       - array of png_text holding image
                            comments
           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                        1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be NULL or empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (NULL or
                                empty for unknown).
           text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                                or empty for unknown).
           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist
           when the library is built with iTXt chunk support.

           num_text       - number of comments

           png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
              num_spalettes);
           palette_ptr    - array of png_sPLT_struct structures
                            to be added to the list of palettes
                            in the info structure.
           num_spalettes  - number of palette structures to be
                            added.

           png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
               unit_type);
           offset_x  - positive offset from the left
                            edge of the screen
           offset_y  - positive offset from the top
                            edge of the screen
           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
               unit_type);
           res_x       - pixels/unit physical resolution
                         in x direction
           res_y       - pixels/unit physical resolution
                         in y direction
           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

           png_set_sCAL(png_ptr, info_ptr, unit, width, height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                         (width and height are doubles)

           png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
           unit        - physical scale units (an integer)
           width       - width of a pixel in physical scale units
           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
              num_unknowns)
           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position to write chunk in file
                                  0: do not write chunk
                                  PNG_HAVE_IHDR: before PLTE
                                  PNG_HAVE_PLTE: before IDAT
                                  PNG_AFTER_IDAT: after IDAT

       The  "location"  member  is set automatically according to what part of
       the output file has already been written.  You  can  change  its  value
       after  calling  png_set_unknown_chunks()  as demonstrated in pngtest.c.
       Within each of the "locations", the chunks are sequenced  according  to
       their  position  in  the structure (that is, the value of "i", which is
       the order in which the chunk was either read from  the  input  file  or
       defined with png_set_unknown_chunks).

       A  quick  word  about  text and num_text.  text is an array of png_text
       structures.  num_text is the number of valid structures in  the  array.
       Each png_text structure holds a language code, a keyword, a text value,
       and a compression type.

       The compression types have the same valid numbers  as  the  compression
       types  of  the  image  data.  Currently, the only valid number is zero.
       However, you can store text either compressed or  uncompressed,  unlike
       images,  which  always have to be compressed.  So if you don't want the
       text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
       Because  tEXt and zTXt chunks don't have a language field, if you spec-
       ify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language
       code or translated keyword will not be written out.

       Until  text  gets  around  1000  bytes, it is not worth compressing it.
       After the text has been written out to the file, the  compression  type
       is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
       so that it isn't written out again at the end (in case you are  calling
       png_write_end() with the same struct.

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image
           Author           Name of image's creator
           Description      Description of image (possibly long)
           Copyright        Copyright notice
           Creation Time    Time of original image creation
                            (usually RFC 1123 format, see below)
           Software         Software used to create the image
           Disclaimer       Legal disclaimer
           Warning          Warning of nature of content
           Source           Device used to create the image
           Comment          Miscellaneous comment; conversion
                            from other image format

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions of what the comment is about.  Some typical  keywords  are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the  image and some after.  For example, you may want to put a descrip-
       tion of the image before the image,  but  leave  the  disclaimer  until
       after, so viewers working over modem connections don't have to wait for
       the disclaimer to go over the modem before they start seeing the image.
       Finally,  keywords  should  be full words, not abbreviations.  Keywords
       and text are in the ISO 8859-1 (Latin-1) character set (a  superset  of
       regular  ASCII) and can not contain NUL characters, and should not con-
       tain control or other unprintable characters.   To  make  the  comments
       widely  readable,  stick  with  basic ASCII, and avoid machine specific
       character set extensions like the IBM-PC character  set.   The  keyword
       must  be  present,  but  you  can leave off the text string on non-com-
       pressed pairs.  Compressed pairs must have a text string, as  only  the
       text  string is compressed anyway, so the compression would be meaning-
       less.

       PNG supports modification time via the png_time structure.  Two conver-
       sion  routines  are  provided, png_convert_from_time_t() for time_t and
       png_convert_from_struct_tm() for struct tm.  The  time_t  routine  uses
       gmtime().   You  don't  have to use either of these, but if you wish to
       fill in the png_time structure directly, you should provide the time in
       universal time (GMT) if possible instead of your local time.  Note that
       the year number is the full year (e.g. 1998, rather than 98  -  PNG  is
       year 2000 compliant!), and that months start with 1.

       If  you  want  to  store  the  time of the original image creation, you
       should use a plain tEXt chunk with the "Creation Time"  keyword.   This
       is  necessary  because  the  "creation time" of a PNG image is somewhat
       vague, depending on whether you mean the PNG file, the time  the  image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.  In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk use RFC 1123 format dates (e.g.  "22  May  1997  18:07:10  GMT"),
       although  this  isn't  a requirement.  Unlike the tIME chunk, the "Cre-
       ation Time" tEXt chunk is not expected to be automatically  changed  by
       the  software.   To  facilitate  the  use of RFC 1123 dates, a function
       png_convert_to_rfc1123(png_timep) is provided to convert from PNG  time
       to an RFC 1123 format string.


   Writing unknown chunks
       You  can use the png_set_unknown_chunks function to queue up chunks for
       writing.  You give it a chunk name, raw data, and a  size;  that's  all
       there  is  to  it.   The  chunks  will be written by the next following
       png_write_info_before_PLTE, png_write_info, or png_write_end  function.
       Any chunks previously read into the info structure's unknown-chunk list
       will also be written out in a sequence that satisfies the PNG  specifi-
       cation's ordering rules.


   The high-level write interface
       At  this  point  there  are two ways to proceed; through the high-level
       write interface, or through a sequence of low-level  write  operations.
       You  can  use the high-level interface if your image data is present in
       the info structure.  All defined output transformations are  permitted,
       enabled by the following masks.

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                             bytes (deprecated).
           PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                             filler bytes
           PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                             filler bytes

       If  you  have  valid  image  data  in  the  info structure (you can use
       png_set_rows() to put image data in  the  info  structure),  simply  do
       this:

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_write_info(),  followed the set of transformations indicated by the
       transform mask, then png_write_image(), and finally png_write_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point  to  transformation  parameters  required  by  some future output
       transform.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_write_png().


   The low-level write interface
       If  you  are  going  the  low-level route instead, you are now ready to
       write all the file information up to the actual  image  data.   You  do
       this with a call to png_write_info().

           png_write_info(png_ptr, info_ptr);

       Note  that  there  is  one  transformation  you  may  need to do before
       png_write_info().  In PNG files, the alpha channel in an image  is  the
       level of opacity.  If your data is supplied as a level of transparency,
       you can invert the alpha channel before you write  it,  so  that  0  is
       fully  transparent  and  255 (in 8-bit or paletted images) or 65535 (in
       16-bit images) is fully opaque, with

           png_set_invert_alpha(png_ptr);

       This must appear before png_write_info()  instead  of  later  with  the
       other  transformations  because in the case of paletted images the tRNS
       chunk data has to be inverted before the tRNS  chunk  is  written.   If
       your  image is not a paletted image, the tRNS data (which in such cases
       represents a single color to be rendered as transparent) won't need  to
       be  changed,  and  you  can  safely  do  this transformation after your
       png_write_info() call.

       If you need to write a private chunk that you want to appear before the
       PLTE  chunk  when  PLTE  is  present, you can write the PNG info in two
       steps, and insert code to write your own chunk between them:

           png_write_info_before_PLTE(png_ptr, info_ptr);
           png_set_unknown_chunks(png_ptr, info_ptr, ...);
           png_write_info(png_ptr, info_ptr);

       After you've written the file information, you can set up  the  library
       to  handle  any special transformations of the image data.  The various
       ways to transform the data will be described in  the  order  that  they
       should  occur.   This  is  important, as some of these change the color
       type and/or bit depth of the data, and some others only work on certain
       color  types and bit depths.  Even though each transformation checks to
       see if it has data that it can do something with, you should make  sure
       to  only enable a transformation if it will be valid for the data.  For
       example, don't swap red and blue on grayscale data.

       PNG files store RGB pixels packed into 3 or 6 bytes.  This  code  tells
       the library to strip input data that has 4 or 8 bytes per pixel down to
       3 or 6 bytes (or strip 2 or 4-byte grayscale+filler  data  to  1  or  2
       bytes per pixel).

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

       where  the 0 is unused, and the location is either PNG_FILLER_BEFORE or
       PNG_FILLER_AFTER, depending upon whether the filler byte in  the  pixel
       is stored XRGB or RGBX.

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       If  the data is supplied at 1 pixel per byte, use this code, which will
       correctly pack the pixels into a single byte:

           png_set_packing(png_ptr);

       PNG files reduce possible bit depths to 1, 2, 4, 8, and  16.   If  your
       data is of another bit depth, you can write an sBIT chunk into the file
       so that decoders can recover the original data if desired.

           /* Set the true bit depth of the image data */
           if (color_type & PNG_COLOR_MASK_COLOR)
           {
               sig_bit.red = true_bit_depth;
               sig_bit.green = true_bit_depth;
               sig_bit.blue = true_bit_depth;
           }
           else
           {
               sig_bit.gray = true_bit_depth;
           }
           if (color_type & PNG_COLOR_MASK_ALPHA)
           {
               sig_bit.alpha = true_bit_depth;
           }

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

       If the data is stored in the row buffer in a bit depth other  than  one
       supported  by  PNG  (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
       this will scale the values to appear to be the correct bit depth as  is
       required by PNG.

           png_set_shift(png_ptr, &sig_bit);

       PNG  files  store  16 bit pixels in network byte order (big-endian, ie.
       most significant bits first).  This code would be used if they are sup-
       plied  the other way (little-endian, i.e. least significant bits first,
       the way PCs store them):

           if (bit_depth > 8)
              png_set_swap(png_ptr);

       If you are using packed-pixel images (1, 2, or 4 bits/pixel),  and  you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       PNG  files  store  3 color pixels in red, green, blue order.  This code
       would be used if they are supplied as blue, green, red:

           png_set_bgr(png_ptr);

       PNG files describe monochrome as black being zero and white being  one.
       This  code  would be used if the pixels are supplied with this reversed
       (black being one and white being zero):

           png_set_invert_mono(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

           void write_transform_fn(png_structp png_ptr, png_row_infop
              row_info, png_bytep data)

       See pngtest.c for a working example.   Your  function  will  be  called
       before any of the other transformations are processed.

       You can also set up a pointer to a user structure for use by your call-
       back function.

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

       The user_channels  and  user_depth  parameters  of  this  function  are
       ignored when writing; you can set them to zero as shown.

       You  can  retrieve  the  pointer  via  the function png_get_user_trans-
       form_ptr().  For example:

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       It is possible to have libpng flush any pending  output,  either  manu-
       ally,  or automatically after a certain number of lines have been writ-
       ten.  To flush the output stream a single time call:

           png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of scanlines have been written, call:

           png_set_flush(png_ptr, nrows);

       Note   that   the   distance   between  rows  is  from  the  last  time
       png_write_flush() was called, or the first row of the image if  it  has
       never  been  called.   So if you write 50 lines, and then png_set_flush
       25, it will flush the output on the next scanline, and every  25  lines
       thereafter,  unless  png_write_flush()  is  called before 25 more lines
       have been written.  If nrows is too small (less than about 10 lines for
       a  640 pixel wide RGB image) the image compression may decrease notice-
       ably (although this may  be  acceptable  for  real-time  applications).
       Infrequent  flushing will only degrade the compression performance by a
       few percent over images that do not use flushing.


   Writing the image data
       That's it for the transformations.  Now you can write the  image  data.
       The  simplest  way to do this is in one function call.  If you have the
       whole image in memory, you can just call png_write_image()  and  libpng
       will write the image.  You will need to pass in an array of pointers to
       each row.  This function  automatically  handles  interlacing,  so  you
       don't  need  to call png_set_interlace_handling() or call this function
       multiple  times,  or  any  of   that   other   stuff   necessary   with
       png_write_rows().

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If  you  don't  want  to  write  the  whole  image at once, you can use
       png_write_rows() instead.  If the file is not interlaced, this is  sim-
       ple:

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       row_pointers is the same as in the png_write_image() call.

       If  you are just writing one row at a time, you can do this with a sin-
       gle row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

       When the file is interlaced, things can get a good  deal  more  compli-
       cated.   The  only  currently (as of the PNG Specification version 1.2,
       dated July 1999) defined  interlacing  scheme  for  PNG  files  is  the
       "Adam7"  interlace scheme, that breaks down an image into seven smaller
       images of varying size.  libpng will build these images for you, or you
       can  do them yourself.  If you want to build them yourself, see the PNG
       specification for details of which pixels to write when.

       If you don't want libpng to handle the interlacing  details,  just  use
       png_set_interlace_handling() and call png_write_rows() the correct num-
       ber of times to write all seven sub-images.

       If you want libpng to build the sub-images, call this before you  start
       writing any rows:

           number_of_passes =
              png_set_interlace_handling(png_ptr);

       This  will  return  the  number  of  passes needed.  Currently, this is
       seven, but may change if another interlace type is added.

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       As some of these rows are not used, and thus  return  immediately,  you
       may  want  to read about interlacing in the PNG specification, and only
       update the rows that are actually used.


   Finishing a sequential write
       After you are finished writing the image, you should finish writing the
       file.   If  you  are interested in writing comments or time, you should
       pass an appropriately filled png_info pointer.  If you are  not  inter-
       ested, you can pass NULL.

           png_write_end(png_ptr, info_ptr);

       When you are done, you can free all memory used by libpng like this:

           png_destroy_write_struct(&png_ptr, &info_ptr);

       It  is  also  possible  to  individually free the info_ptr members that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)
           mask  - identifies data to be freed, a mask
                   containing the bitwise OR of one or
                   more of
                     PNG_FREE_PLTE, PNG_FREE_TRNS,
                     PNG_FREE_HIST, PNG_FREE_ICCP,
                     PNG_FREE_PCAL, PNG_FREE_ROWS,
                     PNG_FREE_SCAL, PNG_FREE_SPLT,
                     PNG_FREE_TEXT, PNG_FREE_UNKN,
                   or simply PNG_FREE_ALL
           seq   - sequence number of item to be freed
                   (-1 for all items)

       This function may be  safely  called  when  the  relevant  storage  has
       already  been freed, or has not yet been allocated, or was allocated by
       the user  and not by libpng,  and will in those cases do nothing.   The
       "seq"  parameter is ignored if only one item of the selected data type,
       such as PLTE, is allowed.  If "seq" is not -1, and multiple  items  are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       If you allocated data such as a palette that you passed  in  to  libpng
       with  png_set_*,  you  must  not  free it until just before the call to
       png_destroy_write_struct().

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or so that it will free data  that  was  allocated  by  the  user  with
       png_malloc() and passed in via a png_set_*() function, with

           png_data_freer(png_ptr, info_ptr, freer, mask)
           mask   - which data elements are affected
                    same choices as in png_free_data()
           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

       For  example,  to  transfer  responsibility  for  some data from a read
       structure to a write structure, you could use

           png_data_freer(read_ptr, read_info_ptr,
              PNG_USER_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
           png_data_freer(write_ptr, write_info_ptr,
              PNG_DESTROY_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

       thereby briefly reassigning responsibility for freeing to the user  but
       immediately  afterwards  reassigning  it once more to the write_destroy
       function.  Having done this, it would then be safe to destroy the  read
       structure  and  continue  to  use  the PLTE, tRNS, and hIST data in the
       write structure.

       This function only affects data that has already been  allocated.   You
       can  call  this function before calling after the png_set_*() functions
       to control whether the user or png_destroy_*() is supposed to free  the
       data.   When the user assumes responsibility for libpng-allocated data,
       the application must use png_free() to  free  it,  and  when  the  user
       transfers  responsibility  to  libpng  for data that the user has allo-
       cated, the user must have used png_malloc() to allocate it.

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans-
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text  structure  it
       combines  these  members  with the key member, and png_free_data() will
       free only text_ptr.key.  Similarly, if you transfer responsibility  for
       free'ing  text_ptr  from  libpng  to your application, your application
       must not separately free those members.  For a more compact example  of
       writing a PNG image, see the file example.c.


V. Modifying/Customizing libpng:
       There are two issues here.  The first is changing how libpng does stan-
       dard things like memory allocation, input/output, and  error  handling.
       The  second  deals with more complicated things like adding new chunks,
       adding new transformations, and generally changing  how  libpng  works.
       Both  of  those  are  compile-time  issues; that is, they are generally
       determined at the time the code is written, and there is rarely a  need
       to provide the user with a means of changing them.

       Memory allocation, input/output, and error handling

       All  of  the  memory  allocation,  input/output,  and error handling in
       libpng goes through callbacks that are user-settable.  The default rou-
       tines  are  in  pngmem.c,  pngrio.c,  pngwio.c, and pngerror.c, respec-
       tively.  To change these functions, call the appropriate png_set_*_fn()
       function.

       Memory  allocation is done through the functions png_malloc(), png_cal-
       loc(), and png_free().  These currently just call the standard C  func-
       tions.   png_calloc() calls png_malloc() and then png_memset() to clear
       the newly allocated memory to zero.  If your pointers can't access more
       then  64K  at a time, you will want to set MAXSEG_64K in zlib.h.  Since
       it is unlikely that the method of handling memory allocation on a plat-
       form will change between applications, these functions must be modified
       in the library at compile time.  If  you  prefer  to  use  a  different
       method   of   allocating   and  freeing  data,  you  can  use  png_cre-
       ate_read_struct_2() or png_create_write_struct_2() to register your own
       functions  as  described  above.   These  functions also provide a void
       pointer that can be retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

           png_voidp malloc_fn(png_structp png_ptr,
              png_alloc_size_t size);
           void free_fn(png_structp png_ptr, png_voidp ptr);

       Your malloc_fn() must return NULL in case of failure.  The png_malloc()
       function  will normally call png_error() if it receives a NULL from the
       system memory allocator or from your replacement malloc_fn().

       Your free_fn() will never be called with a  NULL  ptr,  since  libpng's
       png_free() checks for NULL before calling free_fn().

       Input/Output  in  libpng  is  done  through png_read() and png_write(),
       which currently just call fread() and fwrite().  The FILE *  is  stored
       in  png_struct  and  is  initialized via png_init_io().  If you wish to
       change the method of I/O, the library supplies callbacks that  you  can
       set  through  the  function png_set_read_fn() and png_set_write_fn() at
       run time, instead of calling the png_init_io() function.   These  func-
       tions  also  provide a void pointer that can be retrieved via the func-
       tion png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

           png_set_write_fn(png_structp write_ptr,
               voidp write_io_ptr, png_rw_ptr write_data_fn,
               png_flush_ptr output_flush_fn);

           voidp read_io_ptr = png_get_io_ptr(read_ptr);
           voidp write_io_ptr = png_get_io_ptr(write_ptr);

       The replacement I/O functions must have prototypes as follows:

           void user_read_data(png_structp png_ptr,
               png_bytep data, png_size_t length);
           void user_write_data(png_structp png_ptr,
               png_bytep data, png_size_t length);
           void user_flush_data(png_structp png_ptr);

       The user_read_data() function is responsible for detecting and handling
       end-of-data errors.

       Supplying  NULL  for the read, write, or flush functions sets them back
       to using the default C stream functions, which  expect  the  io_ptr  to
       point  to  a standard *FILE structure.  It is probably a mistake to use
       NULL for one of write_data_fn and output_flush_fn but not both of them,
       unless you have built libpng with PNG_NO_WRITE_FLUSH defined.  It is an
       error to read from a write stream, and vice versa.

       Error handling in libpng is done through png_error() and png_warning().
       Errors  handled through png_error() are fatal, meaning that png_error()
       should never return to its caller.   Currently,  this  is  handled  via
       setjmp()   and   longjmp()   (unless  you  have  compiled  libpng  with
       PNG_NO_SETJMP, in which case it is handled via  PNG_ABORT()),  but  you
       could  change this to do things like exit() if you should wish, as long
       as your function does not return.

       On non-fatal errors, png_warning() is called to print  a  warning  mes-
       sage,  and  then  control  returns  to  the  calling  code.  By default
       png_error() and png_warning() print a message on stderr  via  fprintf()
       unless  the library is compiled with PNG_NO_CONSOLE_IO defined (because
       you don't want the messages) or PNG_NO_STDIO defined (because fprintf()
       isn't  available).   If  you  wish  to change the behavior of the error
       functions, you will need to set up your own message  callbacks.   These
       functions are normally supplied at the time that the png_struct is cre-
       ated.  It is also possible to redirect errors and warnings to your  own
       replacement  functions  after  png_create_*_struct() has been called by
       calling:

           png_set_error_fn(png_structp png_ptr,
               png_voidp error_ptr, png_error_ptr error_fn,
               png_error_ptr warning_fn);

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

       If NULL is supplied for either error_fn or warning_fn, then the  libpng
       default  function will be used, calling fprintf() and/or longjmp() if a
       problem is encountered.  The replacement error  functions  should  have
       parameters as follows:

           void user_error_fn(png_structp png_ptr,
               png_const_charp error_msg);
           void user_warning_fn(png_structp png_ptr,
               png_const_charp warning_msg);

       The motivation behind using setjmp() and longjmp() is the C++ throw and
       catch exception handling methods.  This makes the code much  easier  to
       write, as there is no need to check every return code of every function
       call.  However, there are some uncertainties about the status of  local
       variables  after  a  longjmp,  so the user may want to be careful about
       doing anything after setjmp returns non-zero besides returning  itself.
       Consult  your compiler documentation for more details.  For an alterna-
       tive approach,  you  may  wish  to  use  the  "cexcept"  facility  (see
       http://cexcept.sourceforge.net), which is illustrated in pngvalid.c and
       in contrib/visupng.


   Custom chunks
       If you need to read or write custom chunks, you may need to get  deeper
       into  the  libpng code.  The library now has mechanisms for storing and
       writing chunks of unknown type; you can even declare callbacks for cus-
       tom  chunks.   However, this may not be good enough if the library code
       itself needs to know about interactions between your chunk and existing
       `intrinsic' chunks.

       If you need to write a new intrinsic chunk, first read the PNG specifi-
       cation. Acquire a first level of understanding of how  it  works.   Pay
       particular  attention  to  the  sections that describe chunk names, and
       look at how other chunks were designed, so you can do things similarly.
       Second,  check  out  the sections of libpng that read and write chunks.
       Try to find a chunk that is similar to yours and use it as a  template.
       More  details can be found in the comments inside the code.  It is best
       to handle private or unknown chunks in a generic method,  via  callback
       functions,  instead  of  by  modifying libpng functions. This is illus-
       trated in pngtest.c, which uses a callback function to handle a private
       "vpAg"  chunk  and  the  new  "sTER"  chunk,  which are both unknown to
       libpng.

       If you wish to write your own transformation for the data, look through
       the  part of the code that does the transformations, and check out some
       of the simpler ones to get an idea of how they work.   Try  to  find  a
       similar  transformation  to the one you want to add and copy off of it.
       More details can be found in the comments inside the code itself.


   Configuring for 16 bit platforms
       You will want to look into zconf.h to tell zlib (and thus libpng)  that
       it  cannot allocate more then 64K at a time.  Even if you can, the mem-
       ory won't be accessible.  So limit zlib and libpng to 64K  by  defining
       MAXSEG_64K.


   Configuring for DOS
       For  DOS users who only have access to the lower 640K, you will have to
       limit zlib's memory usage via a  png_set_compression_mem_level()  call.
       See zlib.h or zconf.h in the zlib library for more information.


   Configuring for Medium Model
       Libpng's  support for medium model has been tested on most of the popu-
       lar compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
       defined,  and  FAR  gets defined to far in pngconf.h, and you should be
       all set.  Everything in the library (except for  zlib's  structure)  is
       expecting  far data.  You must use the typedefs with the p or pp on the
       end for pointers (or at least look at them and be careful).  Make  note
       that  the  rows of data are defined as png_bytepp, which is an unsigned
       char far * far *.


   Configuring for gui/windowing platforms:
       You will need to write new error and warning functions that use the GUI
       interface,  as  described  previously, and set them to be the error and
       warning functions at the time that png_create_*_struct() is called,  in
       order to have them available during the structure initialization.  They
       can be changed later via png_set_error_fn().  On  some  compilers,  you
       may also have to change the memory allocators (png_malloc, etc.).


   Configuring for compiler xxx:
       All  includes  for libpng are in pngconf.h.  If you need to add, change
       or delete an include, this is the place to do it.   The  includes  that
       are  not  needed  outside libpng are placed in pngpriv.h, which is only
       used by the routines inside libpng itself.  The files in libpng  proper
       only include pngpriv.h and png.h, which in turn includes pngconf.h.


   Configuring zlib:
       There  are special functions to configure the compression.  Perhaps the
       most useful one changes the compression  level,  which  currently  uses
       input compression values in the range 0 - 9.  The library normally uses
       the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests  have
       shown  that  for  a large majority of images, compression values in the
       range 3-6 compress nearly as well as higher  levels,  and  do  so  much
       faster.   For  online  applications it may be desirable to have maximum
       speed (Z_BEST_SPEED = 1).  With versions of zlib after v0.99,  you  can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre-
       ate files larger than just storing the raw bitmap.  You can specify the
       compression level by calling:

           png_set_compression_level(png_ptr, level);

       Another  useful  one is to reduce the memory level used by the library.
       The memory level defaults to 8, but it can be lowered if you are  short
       on  memory  (running DOS, for example, where you only have 640K).  Note
       that the memory level does have an effect on compression;  among  other
       things,  lower  levels  will  result in sections of incompressible data
       being emitted in smaller stored blocks, with a  correspondingly  larger
       relative overhead of up to 15% in the worst case.

           png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended
       for normal use and may result in writing  an  invalid  PNG  file.   See
       zlib.h for more information on what these mean.

           png_set_compression_strategy(png_ptr,
               strategy);
           png_set_compression_window_bits(png_ptr,
               window_bits);
           png_set_compression_method(png_ptr, method);
           png_set_compression_buffer_size(png_ptr, size);


   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which fil-
       ters are used, and how it goes about picking row filters, you can  call
       one of these functions.  The selection and configuration of row filters
       can have a significant impact on the size  and  encoding  speed  and  a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled by default for RGB  and  grayscale  images  (with  and  without
       alpha),  but not for paletted images nor for any images with bit depths
       less than 8 bits/pixel.

       The 'method' parameter sets the main filtering method,  which  is  cur-
       rently  only '0' in the PNG 1.2 specification.  The 'filters' parameter
       sets which filter(s), if any, should be used for each scanline.  Possi-
       ble  values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on
       and off, respectively.

       Individual filter types are PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FIL-
       TER_UP,  PNG_FILTER_AVG,  PNG_FILTER_PAETH,  which  can be bitwise ORed
       together with '|' to specify one or more filters to use.  These filters
       are  described  in more detail in the PNG specification.  If you intend
       to change the filter type during the course of writing the  image,  you
       should start with flags set for all of the filters you intend to use so
       that libpng can initialize its internal  structures  appropriately  for
       all  of  the  filter  types.   (Note that this means the first row must
       always be adaptively filtered, because libpng currently does not  allo-
       cate  the  filter buffers until png_write_row() is called for the first
       time.)

           filters = PNG_FILTER_NONE | PNG_FILTER_SUB
                     PNG_FILTER_UP | PNG_FILTER_AVG |
                     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

           png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
              filters);
                     The second parameter can also be
                     PNG_INTRAPIXEL_DIFFERENCING if you are
                     writing a PNG to be embedded in a MNG
                     datastream.  This parameter must be the
                     same as the value of filter_method used
                     in png_set_IHDR().


   Removing unwanted object code
       There are a bunch of #define's in pngconf.h that control what parts  of
       libpng  are  compiled.   All the defines end in _SUPPORTED.  If you are
       never going to use a capability, you can change the #define  to  #undef
       before recompiling libpng and save yourself code and data space, or you
       can turn off individual  capabilities  with  defines  that  begin  with
       PNG_NO_.

       You  can  also turn all of the transforms and ancillary chunk capabili-
       ties off en masse with compiler directives that  define  PNG_NO_READ[or
       WRITE]_TRANSFORMS,  or  PNG_NO_READ[or  WRITE]_ANCILLARY_CHUNKS, or all
       four, along with directives to turn on any of the capabilities that you
       do  want.   The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the
       extra transformations but still leave  the  library  fully  capable  of
       reading  and writing PNG files with all known public chunks. Use of the
       PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS  directive  produces  a  library
       that  is  incapable of reading or writing ancillary chunks.  If you are
       not using the progressive reading capability, you  can  turn  that  off
       with  PNG_NO_PROGRESSIVE_READ  (don't confuse this with the INTERLACING
       capability, which you'll still have).

       All the reading and writing specific code are in separate files, so the
       linker  should  only  grab the files it needs.  However, if you want to
       make sure, or if you are building a stand alone library, all the  read-
       ing  files  start  with  "pngr"  and  all  the writing files start with
       "pngw".  The files that don't match  either  (like  png.c,  pngtrans.c,
       etc.)   are  used  for  both reading and writing, and always need to be
       included.  The progressive reader is in pngpread.c

       If you are creating or distributing a dynamically linked library (a .so
       or  DLL  file),  you  should  not  remove  or  disable any parts of the
       library, as this will cause applications linked with different versions
       of  the  library  to  fail if they call functions not available in your
       library.  The size of the  library  itself  should  not  be  an  issue,
       because  only those sections that are actually used will be loaded into
       memory.


   Requesting debug printout
       The macro definition PNG_DEBUG can be used to request debugging  print-
       out.   Set  it to an integer value in the range 0 to 3.  Higher numbers
       result in increasing amounts of debugging information.  The information
       is  printed to the "stderr" file, unless another file name is specified
       in the PNG_DEBUG_FILE macro definition.

       When PNG_DEBUG > 0, the following functions (macros) become available:

          png_debug(level, message)
          png_debug1(level, message, p1)
          png_debug2(level, message, p1, p2)

       in which "level" is compared to PNG_DEBUG to decide  whether  to  print
       the  message,  "message"  is the formatted string to be printed, and p1
       and p2 are parameters that are to be embedded in the  string  according
       to printf-style formatting directives.  For example,

          png_debug1(2, "foo=%d", foo);

       is expanded to

          if(PNG_DEBUG > 2)
            fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

       When  PNG_DEBUG  is defined but is zero, the macros aren't defined, but
       you can still use PNG_DEBUG to control your own debugging:

          #ifdef PNG_DEBUG
              fprintf(stderr, ...
          #endif

       When PNG_DEBUG = 1, the macros are defined, but only  png_debug  state-
       ments  having  level = 0 will be printed.  There aren't any such state-
       ments in this version of libpng, but if you insert some  they  will  be
       printed.


VI. MNG support
       The  MNG  specification  (available  at  http://www.libpng.org/pub/mng)
       allows certain extensions to PNG for PNG images that  are  embedded  in
       MNG  datastreams.   Libpng  can  support  some of these extensions.  To
       enable them, use the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)
          mask is a png_uint_32 containing the bitwise OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES
          feature_set is a png_uint_32 that is the bitwise AND of
             your mask with the set of MNG features that is
             supported by the version of libpng that you are using.

       It is an error to use this function when reading or  writing  a  stand-
       alone  PNG file with the PNG 8-byte signature.  The PNG datastream must
       be wrapped in a MNG datastream.  As a minimum, it  must  have  the  MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support for these or any other MNG chunks; your application  must  pro-
       vide  its  own support for them.  You may wish to consider using libmng
       (available at http://www.libmng.com) instead.


VII. Changes to Libpng from version 0.88
       It should be noted that versions of libpng later than 0.96 are not dis-
       tributed  by  the  original libpng author, Guy Schalnat, nor by Andreas
       Dilger, who had taken over from Guy during 1996 and 1997, and  distrib-
       uted  versions  0.89  through 0.96, but rather by another member of the
       original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas  are  still
       alive and well, but they have moved on to other things.

       The    old    libpng   functions   png_read_init(),   png_write_init(),
       png_info_init(), png_read_destroy(), and png_write_destroy() have  been
       moved  to  PNG_INTERNAL in version 0.95 to discourage their use.  These
       functions will be removed from libpng version 2.0.0.

       The preferred method of creating and initializing the libpng structures
       is  via  the  png_create_read_struct(),  png_create_write_struct(), and
       png_create_info_struct() because they isolate the size  of  the  struc-
       tures  from  the  application,  allow  version error checking, and also
       allow the use of custom error handling routines during the  initializa-
       tion, which the old functions do not.  The functions png_read_destroy()
       and png_write_destroy() do not actually free  the  memory  that  libpng
       allocated  for  these  structs,  but just reset the data structures, so
       they   can   be   used   instead   of   png_destroy_read_struct()   and
       png_destroy_write_struct()  if  you feel there is too much system over-
       head allocating and freeing the png_struct for each image read.

       Setting   the   error   callbacks   via   png_set_message_fn()   before
       png_read_init()  as was suggested in libpng-0.88 is no longer supported
       because this caused applications that do not use custom error functions
       to fail if the png_ptr was not initialized to zero.  It is still possi-
       ble to set the error callbacks AFTER png_read_init(), or to change them
       with  png_set_error_fn(),  which  is essentially the same function, but
       with a new name to force compilation errors with applications that  try
       to use the old method.

       Support  for  the  sCAL,  iCCP,  iTXt,  and  sPLT  chunks  was added at
       libpng-1.0.6; however, iTXt support was not enabled by default.

       Starting with version 1.0.7, you can find  out  which  version  of  the
       library you are using at run-time:

          png_uint_32 libpng_vn = png_access_version_number();

       The  number libpng_vn is constructed from the major version, minor ver-
       sion with leading zero, and release number with  leading  zero,  (e.g.,
       libpng_vn for version 1.0.7 is 10007).

       You  can also check which version of png.h you used when compiling your
       application:

          png_uint_32 application_vn = PNG_LIBPNG_VER;


VIII. Changes to Libpng from version 1.0.x to 1.2.x
       Support for user memory management was enabled by default.   To  accom-
       plish   this,   the   functions   png_create_read_struct_2(),  png_cre-
       ate_write_struct_2(),  png_set_mem_fn(),  png_get_mem_ptr(),   png_mal-
       loc_default(), and png_free_default() were added.

       Support  for  the  iTXt chunk has been enabled by default as of version
       1.2.41.

       Support for certain MNG features was enabled.

       Support for numbered error messages was added.  However, we  never  got
       around   to  actually  numbering  the  error  messages.   The  function
       png_set_strip_error_numbers() was added (Note: the prototype  for  this
       function  was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
       builds of libpng-1.2.15.  It was restored in libpng-1.2.36).

       The png_malloc_warn() function was added at libpng-1.2.3.  This  issues
       a  png_warning  and  returns  NULL instead of aborting when it fails to
       acquire the requested memory allocation.

       Support for setting user limits on image width and height  was  enabled
       by       default.        The      functions      png_set_user_limits(),
       png_get_user_width_max(), and png_get_user_height_max() were  added  at
       libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The    function    png_set_expand_gray_1_2_4_to_8()    was   added   at
       libpng-1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the new function  does
       not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() func-
       tion is deprecated.

       A number of macro definitions in support of runtime selection of assem-
       bler  code  features  (especially Intel MMX code support) were added at
       libpng-1.2.0:

           PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
           PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
           PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
           PNG_ASM_FLAG_MMX_READ_INTERLACE
           PNG_ASM_FLAG_MMX_READ_FILTER_SUB
           PNG_ASM_FLAG_MMX_READ_FILTER_UP
           PNG_ASM_FLAG_MMX_READ_FILTER_AVG
           PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
           PNG_ASM_FLAGS_INITIALIZED
           PNG_MMX_READ_FLAGS
           PNG_MMX_FLAGS
           PNG_MMX_WRITE_FLAGS
           PNG_MMX_FLAGS

       We added the following functions in support  of  runtime  selection  of
       assembler code features:

           png_get_mmx_flagmask()
           png_set_mmx_thresholds()
           png_get_asm_flags()
           png_get_mmx_bitdepth_threshold()
           png_get_mmx_rowbytes_threshold()
           png_set_asm_flags()

       We  replaced all of these functions with simple stubs in libpng-1.2.20,
       when the Intel assembler code was removed due to a licensing issue.

       These macros are deprecated:

           PNG_READ_TRANSFORMS_NOT_SUPPORTED
           PNG_PROGRESSIVE_READ_NOT_SUPPORTED
           PNG_NO_SEQUENTIAL_READ_SUPPORTED
           PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
           PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
           PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

           PNG_NO_READ_TRANSFORMS
           PNG_NO_PROGRESSIVE_READ
           PNG_NO_SEQUENTIAL_READ
           PNG_NO_WRITE_TRANSFORMS
           PNG_NO_READ_ANCILLARY_CHUNKS
           PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been deprecated
       since libpng-1.0.16 and libpng-1.2.6.

       The function
           png_check_sig(sig, num) was replaced with
           !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
           png_set_gray_1_2_4_to_8()  which  also  expands  tRNS  to alpha was
       replaced with
           png_set_expand_gray_1_2_4_to_8() which does not. It has been depre-
       cated since libpng-1.0.18 and 1.2.9.


IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
       Private  libpng  prototypes and macro definitions were moved from png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions     png_set_benign_errors(),     png_benign_error(),      and
       png_chunk_benign_error() were added.

       Support  for  setting the maximum amount of memory that the application
       will allocate for reading chunks was added, as a security measure.  The
       functions  png_set_chunk_cache_max() and png_get_chunk_cache_max() were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and   functions   png_get_io_chunk_name()   and  png_get_io_state()  in
       pngget.c

       We added PNG_TRANSFORM_GRAY_TO_RGB to the  available  high-level  input
       transforms.

       Checking  for  and  reporting of errors in the IHDR chunk is more thor-
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
          #define png_voidp_NULL            (png_voidp)NULL  were  eliminated.
       If you used these in your application, just use NULL instead.

       The  png_struct and info_struct members "trans" and "trans_values" were
       changed to "trans_alpha" and "trans_color", respectively.

       The obsolete, unused pnggccrd.c and pngvcrd.c files and  related  make-
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The    functions   png_read_init(info_ptr),   png_write_init(info_ptr),
       png_info_init(info_ptr),  png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The  png_permit_empty_plte()  was removed. It has been deprecated since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We  removed  the  obsolete   stub   functions   png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit-
       depth_threshold(),                    png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We   removed  the  obsolete  png_check_sig(),  png_memcpy_check(),  and
       png_memset_check() functions.   Instead  use  !png_sig_cmp(),  png_mem-
       cpy(), and png_memset(), respectively.

       The  function png_set_gray_1_2_4_to_8() was removed. It has been depre-
       cated  since  libpng-1.0.18  and  1.2.9,  when  it  was  replaced  with
       png_set_expand_gray_1_2_4_to_8()   because  the  former  function  also
       expanded palette images.

       Macros for png_get_uint_16, png_get_uint_32,  and  png_get_int_32  were
       added  and  are used by default instead of the corresponding functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but   not   the   function)  incorrectly  returned  a  value  of  type
       png_uint_32.

       We changed the prototype for png_malloc() from
           png_malloc(png_structp png_ptr, png_uint_32 size) to
           png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This also applies to  the  prototype  for  the  user  replacement  mal-
       loc_fn().

       The  png_calloc()  function  was  added  and  is  used  in  place of of
       "png_malloc(); png_memset();" except  in  the  case  in  png_read_png()
       where the array consists of pointers; in this case a "for" loop is used
       after the png_malloc() to set the pointers to  NULL,  to  give  robust.
       behavior  in  case  the application runs out of memory part-way through
       the process.

       We changed  the  prototypes  of  png_get_compression_buffer_size()  and
       png_set_compression_buffer_size()  to  work  with png_size_t instead of
       png_uint_32.

       Support for numbered error messages was removed by  default,  since  we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers() was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc()  function  no  longer  zeroes out the memory that it allo-
       cates.

       Support for dithering was disabled by default in libpng-1.4.0,  because
       it  has  not  been well tested and doesn't actually "dither".  The code
       was not removed, however, and could be enabled by building libpng  with
       PNG_READ_DITHER_SUPPORTED  defined.   In libpng-1.4.2, this support was
       re-enabled, but the function was renamed png_set_quantize() to  reflect
       more  accurately  what  it  actually  does.   At  the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to  PNG_QUAN-
       TIZE_[RED,GREEN,BLUE]_BITS.

       We removed the trailing '.' from the warning and error messages.

       Starting  with  libpng-1.4.17,  attempting  to  set an over-length PLTE
       chunk is an error. Previously this requirement of the PNG specification
       was not enforced, and the palette was always limited to 256 entries. An
       over-length PLTE chunk found in an input PNG is silently truncated.


X. Detecting libpng
       The png_get_io_ptr() function has been present since  libpng-0.88,  has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres-
       ence  of  any libpng version since 0.88.  In an autoconf "configure.in"
       you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...


XI. Source code repository
       Since about February 2009, version 1.2.34, libpng has been under  "git"
       source  control.   The  git  repository  was  built  from  old  libpng-
       x.y.z.tar.gz files going back to version 0.70.  You can access the  git
       repository (read only) at

           git://git.code.sf.net/p/libpng/code

       or  you can browse it with a web browser by selecting the "code" button
       at

           https://sourceforge.net/projects/libpng/

       Patches can be sent to glennrp at users.sourceforge.net or to  png-mng-
       implement at lists.sourceforge.net or you can upload them to the libpng
       bug tracker at

           http://libpng.sourceforge.net


XII. Coding style
       Our  coding   style   is   similar   to   the   "Allman"   style   (See
       http://en.wikipedia.org/wiki/Indent_style#Allman_style),   with   curly
       braces on separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

       The braces can be omitted from simple one-line actions:

           if (condition)
              return (0);

       We use 3-space indentation, except for continued statements  which  are
       usually  indented the same as the first line of the statement plus four
       more spaces.

       For macro definitions we use 2-space indentation,  always  leaving  the
       "#" in the first column.

           #ifndef PNG_NO_FEATURE
           #  ifndef PNG_FEATURE_SUPPORTED
           #    define PNG_FEATURE_SUPPORTED
           #  endif
           #endif

       Comments  appear  with  the leading "/*" at the same indentation as the
       statement that follows the comment:

           /* Single-line comment */
           statement;

           /* This is a multiple-line
            * comment.
            */
           statement;

       Very short comments can be placed after the end  of  the  statement  to
       which they pertain:

           statement;    /* comment */

       We  don't use C++ style ("//") comments. We have, however, used them in
       the past in some now-abandoned MMX assembler code.

       Functions and their curly braces are not indented, and  exported  func-
       tions are marked with PNGAPI:

        /* This is a public function that is visible to
         * application programers. It does thus-and-so.
         */
        void PNGAPI
        png_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The  prototypes  for  all exported functions appear in png.h, above the
       comment that says

           /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

        void /* PRIVATE */
        png_non_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ and in libpngpf.3 */

       To  avoid  polluting  the  global  namespace, the names of all exported
       functions and variables begin with "png_", and all publicly  visible  C
       preprocessor  macros  begin  with  "PNG".  We request that applications
       that use libpng *not* begin any of their own  symbols  with  either  of
       these strings.

       We put a space after each comma and after each semicolon in "for" stat-
       ments, and we put spaces before and after each C  binary  operator  and
       after "for" or "while", and before "?".  We don't put a space between a
       typecast and the expression being cast, nor do we  put  one  between  a
       function name and the left parenthesis that follows it:

           for (i = 2; i > 0; --i)
              y[i] = a(x) + (int)b;

       We  prefer  #ifdef  and #ifndef to #if defined() and if !defined() when
       there is only one macro being tested.

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.


XIII. Y2K Compliance in libpng
       Since the PNG Development group is an ad-hoc body,  we  can't  make  an
       official declaration.

       This  is  your  unofficial  assurance that libpng from version 0.71 and
       upward through 1.4.20 are Y2K compliant.  It is my belief that  earlier
       versions were also Y2K compliant.

       Libpng  only  has  three year fields.  One is a 2-byte unsigned integer
       that will hold years up to 65535.  The other two hold the date in  text
       format, and will hold years up to 9999.

       The integer is
           "png_uint_16 year" in png_time_struct.

       The strings are
           "png_charp time_buffer" in png_struct and
           "near_time_buffer", which is a local character string in png.c.

       There are seven time-related functions:

           png_convert_to_rfc_1123() in png.c
             (formerly png_convert_to_rfc_1152() in error)
           png_convert_from_struct_tm() in pngwrite.c, called
             in pngwrite.c
           png_convert_from_time_t() in pngwrite.c
           png_get_tIME() in pngget.c
           png_handle_tIME() in pngrutil.c, called in pngread.c
           png_set_tIME() in pngset.c
           png_write_tIME() in pngwutil.c, called in pngwrite.c

       All appear to handle dates properly in a Y2K environment.  The png_con-
       vert_from_time_t() function calls gmtime() to convert from system clock
       time,  which  returns  (year  - 1900), which we properly convert to the
       full 4-digit year.  There is  a  possibility  that  applications  using
       libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
       function, or that they are incorrectly  passing  only  a  2-digit  year
       instead  of  "year  - 1900" into the png_convert_from_struct_tm() func-
       tion, but this is not under our control.  The libpng documentation  has
       always  stated that it works with 4-digit years, and the APIs have been
       documented as such.

       The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
       integer to hold the year, and can hold years as large as 65535.

       zlib, upon which libpng depends, is also Y2K compliant.  It contains no
       date-related code.


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group


NOTE
       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional  factors  outside the authors' control, version numbering on
       the library has not always been consistent  and  straightforward.   The
       following  table  summarizes matters since version 0.89c, which was the
       first widely used release:

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c "1.0 beta 3"     0.89      89  1.0.89
        0.90  "1.0 beta 4"     0.90      90  0.90  [should have been 2.0.90]
        0.95  "1.0 beta 5"     0.95      95  0.95  [should have been 2.0.95]
        0.96  "1.0 beta 6"     0.96      96  0.96  [should have been 2.0.96]
        0.97b "1.00.97 beta 7" 1.00.97   97  1.0.1 [should have been 2.0.97]
        0.97c                  0.97      97  2.0.97
        0.98                   0.98      98  2.0.98
        0.99                   0.99      98  2.0.99
        0.99a-m                0.99      99  2.0.99
        1.00                   1.00     100  2.1.0 [100 should be 10000]
        1.0.0      (from here on, the   100  2.1.0 [100 should be 10000]
        1.0.1       png.h string is   10001  2.1.0
        1.0.1a-e    identical to the  10002  from here on, the shared library
        1.0.2       source version)   10002  is 2.V where V is the source code
        1.0.2a-b                      10003  version, except as noted.
        1.0.3                         10003
        1.0.3a-d                      10004
        1.0.4                         10004
        1.0.4a-f                      10005
        1.0.5 (+ 2 patches)           10005
        1.0.5a-d                      10006
        1.0.5e-r                      10100 (not source compatible)
        1.0.5s-v                      10006 (not binary compatible)
        1.0.6 (+ 3 patches)           10006 (still binary incompatible)
        1.0.6d-f                      10007 (still binary incompatible)
        1.0.6g                        10007
        1.0.6h                        10007  10.6h (testing xy.z so-numbering)
        1.0.6i                        10007  10.6i
        1.0.6j                         10007   2.1.0.6j   (incompatible   with
       1.0.0)
        1.0.7beta11-14        DLLNUM  10007  2.1.0.7beta11-14 (binary compati-
       ble)
        1.0.7beta15-18           1    10007  2.1.0.7beta15-18 (binary compati-
       ble)
        1.0.7rc1-2               1    10007  2.1.0.7rc1-2 (binary compatible)
        1.0.7                    1    10007  (still compatible)
        ...
        1.0.19                  10    10019  10.so.0.19[.0]
        ...
        1.4.20                  14    10420  14.so.14.20[.0]

       Henceforth  the  source version will match the shared-library minor and
       patch numbers; the shared-library major version number will be used for
       changes   in   backward   compatibility,   as   it  is  intended.   The
       PNG_PNGLIB_VER macro, which is not used within libpng but is  available
       for  applications, is an unsigned integer of the form xyyzz correspond-
       ing to the source version x.y.z (leading zeros in y and z).  Beta  ver-
       sions  were  given  the  previous  public release number plus a letter,
       until version 1.0.6j; from then on they were given the upcoming  public
       release number plus "betaNN" or "rcNN".



ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------------+
       |ATTRIBUTE TYPE |    ATTRIBUTE VALUE     |
       +---------------+------------------------+
       |Availability   | image/library/libpng14 |
       +---------------+------------------------+
       |Stability      | Uncommitted            |
       +---------------+------------------------+
SEE ALSO
       libpngpf(3), png(5)

       libpng:

              http://libpng.sourceforge.net   (follow   the  [DOWNLOAD]  link)
              http://www.libpng.org/pub/png


       zlib:

              (generally) at the same location as libpng or at
              ftp://ftp.info-zip.org/pub/infozip/zlib


       PNGspecification:RFC2083

              (generally) at the same location as libpng or at
              ftp://ftp.rfc-editor.org:/in-notes/rfc2083.txt
              or (as a W3C Recommendation) at
              http://www.w3.org/TR/REC-png.html


       In the case of any inconsistency between the PNG specification and this
       library, the specification takes precedence.


AUTHORS
       This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>

       The  contributing authors would like to thank all those who helped with
       testing, bug fixes, and patience.  This  wouldn't  have  been  possible
       without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng version 1.4.20 - December 29, 2016: Initially created in 1995 by
       Guy Eric Schalnat, then of Group  42,  Inc.   Currently  maintained  by
       Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement  at  lists.sf.net  (subscription required; visit png-
       mng-implement at lists.sourceforge.net  (subscription  required;  visit
       https://lists.sourceforge.net/lists/listinfo/png-mng-implement  to sub-
       scribe).


NOTICES:
       This copy of the libpng notices is provided for your  convenience.   In
       case  of  any discrepancy between this copy and the notices in the file
       png.h that is included in the libpng  distribution,  the  latter  shall
       prevail.

       COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

       If you modify libpng you may insert additional notices immediately fol-
       lowing this sentence.

       This code is released under the libpng license.

       libpng versions 1.0.7, July 1, 2000, through 1.4.20, December 29, 2016,
       are Copyright (c) 2000-2002, 2004, 2006-2016 Glenn Randers-Pehrson, are
       derived from libpng-1.0.6, and are distributed according  to  the  same
       disclaimer  and  license as libpng-1.0.6 with the following individuals
       added to the list of Contributing Authors:

          Simon-Pierre Cadieux
          Eric S. Raymond
          Cosmin Truta
          Gilles Vollant

       and with the following additions to the disclaimer:

          There is no warranty against interference with your enjoyment of the
          library or against infringement.  There is no warranty that our
          efforts or the library will fulfill any of your particular purposes
          or needs.  This library is provided with all faults, and the entire
          risk of satisfactory quality, performance, accuracy, and  effort  is
       with
          the user.

       libpng  versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
       Copyright  (c)  1998-2000  Glenn  Randers-Pehrson,  are  derived   from
       libpng-0.96,  and  are distributed according to the same disclaimer and
       license as libpng-0.96, with the following  individuals  added  to  the
       list of Contributing Authors:

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

       libpng  versions 0.89, June 1996, through 0.96, May 1997, are Copyright
       (c) 1996-1997 Andreas Dilger, are derived  from  libpng-0.88,  and  are
       distributed   according   to   the   same  disclaimer  and  license  as
       libpng-0.88, with the following individuals added to the list  of  Con-
       tributing Authors:

          John Bowler
          Kevin Bracey
          Sam Bushell
          Magnus Holmgren
          Greg Roelofs
          Tom Tanner

       libpng  versions  0.5,  May 1995, through 0.88, January 1996, are Copy-
       right (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.

       For the purposes of this copyright and license, "Contributing  Authors"
       is defined as the following set of individuals:

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

       The  PNG  Reference  Library  is  supplied  "AS  IS".  The Contributing
       Authors and Group  42,  Inc.  disclaim  all  warranties,  expressed  or
       implied, including, without limitation, the warranties of merchantabil-
       ity and of fitness for any purpose.  The Contributing Authors and Group
       42,  Inc.   assume  no liability for direct, indirect, incidental, spe-
       cial, exemplary, or consequential damages, which may  result  from  the
       use of the PNG Reference Library, even if advised of the possibility of
       such damage.

       Permission is hereby granted to use, copy, modify, and distribute  this
       source  code, or portions hereof, for any purpose, without fee, subject
       to the following restrictions:

         1. The origin of this source code must not be misrepresented.

         2. Altered versions must be plainly marked as such and must not
            be misrepresented as being the original source.

         3. This Copyright notice may not be removed or altered from any
            source or altered source distribution.

       The Contributing Authors and Group 42, Inc. specifically permit,  with-
       out  fee,  and  encourage the use of this source code as a component to
       supporting the PNG file format in commercial products.  If you use this
       source  code  in a product, acknowledgment is not required but would be
       appreciated.

       END OF COPYRIGHT NOTICE, DISCLAIMER, and LICENSE.

       A "png_get_copyright" function is  available,  for  convenient  use  in
       "about" boxes and the like:

          printf("%s", png_get_copyright(NULL));

       Also,  the PNG logo (in PNG format, of course) is supplied in the files
       "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).

       Libpng is OSI Certified  Open  Source  Software.   OSI  Certified  Open
       Source  is  a certification mark of the Open Source Initiative. OSI has
       not addressed the additional disclaimers inserted at version 1.0.7.

       Glenn Randers-Pehrson glennrp  at  users.sourceforge.net  December  29,
       2016





NOTES
       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source        was        downloaded        from         https://source-
       forge.net/projects/libpng/files/libpng14/1.4.20/libpng-1.4.20.tar.gz

       Further information about this software can be found on the open source
       community website at http://www.libpng.org/.



                               December 29, 2016                     LIBPNG(3)