java.lang.Object java.awt.AlphaComposite
public final class AlphaComposite
The AlphaComposite class implements basic alpha compositing rules for combining source and destination colors to achieve blending and transparency effects with graphics and images. The specific rules implemented by this class are the basic set of 12 rules described in T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 253-259. The rest of this documentation assumes some familiarity with the definitions and concepts outlined in that paper.
This class extends the standard equations defined by Porter and Duff to include one additional factor. An instance of the AlphaComposite class can contain an alpha value that is used to modify the opacity or coverage of every source pixel before it is used in the blending equations.
It is important to note that the equations defined by the Porter and Duff paper are all defined to operate on color components that are premultiplied by their corresponding alpha components. Since the ColorModel and Raster classes allow the storage of pixel data in either premultiplied or non-premultiplied form, all input data must be normalized into premultiplied form before applying the equations and all results might need to be adjusted back to the form required by the destination before the pixel values are stored.
Also note that this class defines only the equations for combining color and alpha values in a purely mathematical sense. The accurate application of its equations depends on the way the data is retrieved from its sources and stored in its destinations. See Implementation Caveats for further information.
The following factors are used in the description of the blending equation in the Porter and Duff paper:
Factor Definition A s the alpha component of the source pixel C s a color component of the source pixel in premultiplied form A d the alpha component of the destination pixel C d a color component of the destination pixel in premultiplied form F s the fraction of the source pixel that contributes to the output F d the fraction of the destination pixel that contributes to the output A r the alpha component of the result C r a color component of the result in premultiplied form
Using these factors, Porter and Duff define 12 ways of choosing the blending factors F s and F d to produce each of 12 desirable visual effects. The equations for determining F s and F d are given in the descriptions of the 12 static fields that specify visual effects. For example, the description for SRC_OVER specifies that F s = 1 and F d = (1-A s ). Once a set of equations for determining the blending factors is known they can then be applied to each pixel to produce a result using the following set of equations:
Fs = f(Ad) Fd = f(As) Ar = As*Fs + Ad*Fd Cr = Cs*Fs + Cd*Fd
The following factors will be used to discuss our extensions to the blending equation in the Porter and Duff paper:
Factor Definition C sr one of the raw color components of the source pixel C dr one of the raw color components of the destination pixel A ac the "extra" alpha component from the AlphaComposite instance A sr the raw alpha component of the source pixel A dr the raw alpha component of the destination pixel A df the final alpha component stored in the destination C df the final raw color component stored in the destination
The AlphaComposite class defines an additional alpha value that is applied to the source alpha. This value is applied as if an implicit SRC_IN rule were first applied to the source pixel against a pixel with the indicated alpha by multiplying both the raw source alpha and the raw source colors by the alpha in the AlphaComposite. This leads to the following equation for producing the alpha used in the Porter and Duff blending equation:
As = Asr * AacAll of the raw source color components need to be multiplied by the alpha in the AlphaComposite instance. Additionally, if the source was not in premultiplied form then the color components also need to be multiplied by the source alpha. Thus, the equation for producing the source color components for the Porter and Duff equation depends on whether the source pixels are premultiplied or not:
Cs = Csr * Asr * Aac (if source is not premultiplied) Cs = Csr * Aac (if source is premultiplied)No adjustment needs to be made to the destination alpha:
Ad = Adr
The destination color components need to be adjusted only if they are not in premultiplied form:
Cd = Cdr * Ad (if destination is not premultiplied) Cd = Cdr (if destination is premultiplied)
The adjusted A s , A d , C s , and C d are used in the standard Porter and Duff equations to calculate the blending factors F s and F d and then the resulting premultiplied components A r and C r .
The results only need to be adjusted if they are to be stored back into a destination buffer that holds data that is not premultiplied, using the following equations:
Adf = Ar Cdf = Cr (if dest is premultiplied) Cdf = Cr / Ar (if dest is not premultiplied)Note that since the division is undefined if the resulting alpha is zero, the division in that case is omitted to avoid the "divide by zero" and the color components are left as all zeros.
For performance reasons, it is preferrable that Raster objects passed to the compose method of a CompositeContext object created by the AlphaComposite class have premultiplied data. If either the source Raster or the destination Raster is not premultiplied, however, appropriate conversions are performed before and after the compositing operation.
Typically the integer values are related to the floating point values in such a way that the integer 0 is equated to the floating point value 0.0 and the integer 2^n-1 (where n is the number of bits in the representation) is equated to 1.0. For 8-bit representations, this means that 0x00 represents 0.0 and 0xff represents 1.0.
(A, R, G, B) = (0x01, 0xb0, 0x00, 0x00)
If integer math were being used and this value were being composited in SRC mode with no extra alpha, then the math would indicate that the results were (in integer format):
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
Note that the intermediate values, which are always in premultiplied form, would only allow the integer red component to be either 0x00 or 0x01. When we try to store this result back into a destination that is not premultiplied, dividing out the alpha will give us very few choices for the non-premultiplied red value. In this case an implementation that performs the math in integer space without shortcuts is likely to end up with the final pixel values of:
(A, R, G, B) = (0x01, 0xff, 0x00, 0x00)
(Note that 0x01 divided by 0x01 gives you 1.0, which is equivalent to the value 0xff in an 8-bit storage format.)
Alternately, an implementation that uses floating point math might produce more accurate results and end up returning to the original pixel value with little, if any, roundoff error. Or, an implementation using integer math might decide that since the equations boil down to a virtual NOP on the color values if performed in a floating point space, it can transfer the pixel untouched to the destination and avoid all the math entirely.
These implementations all attempt to honor the same equations, but use different tradeoffs of integer and floating point math and reduced or full equations. To account for such differences, it is probably best to expect only that the premultiplied form of the results to match between implementations and image formats. In this case both answers, expressed in premultiplied form would equate to:
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
and thus they would all match.
Field Summary | |
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static AlphaComposite |
Clear
AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f. |
static int |
CLEAR
Both the color and the alpha of the destination are cleared (Porter-Duff Clear rule). |
static AlphaComposite |
Dst
AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f. |
static int |
DST
The destination is left untouched (Porter-Duff Destination rule). |
static int |
DST_ATOP
The part of the destination lying inside of the source is composited over the source and replaces the destination (Porter-Duff Destination Atop Source rule). |
static int |
DST_IN
The part of the destination lying inside of the source replaces the destination (Porter-Duff Destination In Source rule). |
static int |
DST_OUT
The part of the destination lying outside of the source replaces the destination (Porter-Duff Destination Held Out By Source rule). |
static int |
DST_OVER
The destination is composited over the source and the result replaces the destination (Porter-Duff Destination Over Source rule). |
static AlphaComposite |
DstAtop
AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f. |
static AlphaComposite |
DstIn
AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f. |
static AlphaComposite |
DstOut
AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f. |
static AlphaComposite |
DstOver
AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f. |
static AlphaComposite |
Src
AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f. |
static int |
SRC
The source is copied to the destination (Porter-Duff Source rule). |
static int |
SRC_ATOP
The part of the source lying inside of the destination is composited onto the destination (Porter-Duff Source Atop Destination rule). |
static int |
SRC_IN
The part of the source lying inside of the destination replaces the destination (Porter-Duff Source In Destination rule). |
static int |
SRC_OUT
The part of the source lying outside of the destination replaces the destination (Porter-Duff Source Held Out By Destination rule). |
static int |
SRC_OVER
The source is composited over the destination (Porter-Duff Source Over Destination rule). |
static AlphaComposite |
SrcAtop
AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f. |
static AlphaComposite |
SrcIn
AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f. |
static AlphaComposite |
SrcOut
AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f. |
static AlphaComposite |
SrcOver
AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f. |
static AlphaComposite |
Xor
AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f. |
static int |
XOR
The part of the source that lies outside of the destination is combined with the part of the destination that lies outside of the source (Porter-Duff Source Xor Destination rule). |
Method Summary | |
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CompositeContext |
createContext
(
ColorModel
srcColorModel,
ColorModel
dstColorModel,
RenderingHints
hints) Creates a context for the compositing operation. |
AlphaComposite |
derive
(float alpha)
Returns a similar AlphaComposite object that uses the specified alpha value. |
AlphaComposite |
derive
(int rule)
Returns a similar AlphaComposite object that uses the specified compositing rule. |
boolean |
equals
(
Object
obj) Determines whether the specified object is equal to this AlphaComposite. |
float |
getAlpha
() Returns the alpha value of this AlphaComposite. |
static AlphaComposite |
getInstance
(int rule) Creates an AlphaComposite object with the specified rule. |
static AlphaComposite |
getInstance
(int rule, float alpha) Creates an AlphaComposite object with the specified rule and the constant alpha to multiply with the alpha of the source. |
int |
getRule
() Returns the compositing rule of this AlphaComposite. |
int |
hashCode
() Returns the hashcode for this composite. |
Methods inherited from class java.lang. Object |
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clone , finalize , getClass , notify , notifyAll , toString , wait , wait , wait |
Field Detail |
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public static final int CLEAR
F s = 0 and F d = 0, thus:
Ar = 0 Cr = 0
public static final int SRC
F s = 1 and F d = 0, thus:
Ar = As Cr = Cs
public static final int DST
F s = 0 and F d = 1, thus:
Ar = Ad Cr = Cd
public static final int SRC_OVER
F s = 1 and F d = (1-A s ), thus:
Ar = As + Ad*(1-As) Cr = Cs + Cd*(1-As)
public static final int DST_OVER
F s = (1-A d ) and F d = 1, thus:
Ar = As*(1-Ad) + Ad Cr = Cs*(1-Ad) + Cd
public static final int SRC_IN
F s = A d and F d = 0, thus:
Ar = As*Ad Cr = Cs*Ad
public static final int DST_IN
F s = 0 and F d = A s , thus:
Ar = Ad*As Cr = Cd*As
public static final int SRC_OUT
F s = (1-A d ) and F d = 0, thus:
Ar = As*(1-Ad) Cr = Cs*(1-Ad)
public static final int DST_OUT
F s = 0 and F d = (1-A s ), thus:
Ar = Ad*(1-As) Cr = Cd*(1-As)
public static final int SRC_ATOP
F s = A d and F d = (1-A s ), thus:
Ar = As*Ad + Ad*(1-As) = Ad Cr = Cs*Ad + Cd*(1-As)
public static final int DST_ATOP
F s = (1-A d ) and F d = A s , thus:
Ar = As*(1-Ad) + Ad*As = As Cr = Cs*(1-Ad) + Cd*As
public static final int XOR
F s = (1-A d ) and F d = (1-A s ), thus:
Ar = As*(1-Ad) + Ad*(1-As) Cr = Cs*(1-Ad) + Cd*(1-As)
public static final AlphaComposite Clear
public static final AlphaComposite Src
public static final AlphaComposite Dst
public static final AlphaComposite SrcOver
public static final AlphaComposite DstOver
public static final AlphaComposite SrcIn
public static final AlphaComposite DstIn
public static final AlphaComposite SrcOut
public static final AlphaComposite DstOut
public static final AlphaComposite SrcAtop
public static final AlphaComposite DstAtop
public static final AlphaComposite Xor
Method Detail |
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public static AlphaComposite getInstance(int rule)
public static AlphaComposite getInstance(int rule, float alpha)
public CompositeContext createContext(ColorModel srcColorModel, ColorModel dstColorModel, RenderingHints hints)
public float getAlpha()
public int getRule()
public AlphaCompositederive (int rule)
public AlphaCompositederive (float alpha)
public int hashCode()
public boolean equals(Object obj)
The result is true if and only if the argument is not null and is an AlphaComposite object that has the same compositing rule and alpha value as this object.