Implementation Notes and Limitations

Partially Transparent Alpha

Alpha values that are not completely opaque (1) or completely transparent (0) should be used with caution. Compositing operations with partial transparency yield the highest image quality only when a large number of colors are available in the DPS color cube and gray ramp. That is, image quality is best with a 24-bit TrueColor or 8-bit StaticGray visual, and image quality will be poor with an 8-bit PseudoColor visual. In addition, the performance of compositing operations is greatly reduced for partially transparent pixels due to the extra computation required in these cases.

Indexed Color Visuals

For best results with the Highlight op, the number of colors in the DPS context's gray ramp should be such that

fract(((float) numgrays - 1)* 2. / 3.) == 0

In other words, (numgrays = 4, 7, 6, 8, 16, ....). This ensures that the color 2/3 gray is not halftoned.

Given the limited number of colors usually available in the DPS color cube and gray ramp, images with alpha values that are not completely opaque (1) or completely transparent (0) should be avoided to obtain best image quality.

Compositing operations are only defined for pixels values that are in the gray ramp or color cube specified by the gstate. Compositing pixels with values outside the color cube and gray ramp may not yield expected results.

Monochrome Displays

The results of compositing operations for 1-bit drawables that have alpha values that are not equal to 0 or 1 is undefined.

The op Highlight inverts the color of the pixel on a 1-bit drawable.

Interaction with X Drawing Operations

Drawables that have been rendered to with non-opaque alpha have additional pixel storage associated with them, called the alpha channel. X Window system operations do not affect the alpha channel, with the following exceptions:

• When windows with alpha channel are exposed, if the window has an X background defined (background != None), when the background is painted, the alpha component of the exposed pixels is painted with alpha = 1.

• When a window is resized, the alpha channel storage is resized.

Destroying the Alpha Channel

The erasepage operator paints the current drawable of the graphics state with opaque white. Thus, the alpha values for all pixels in the drawable are equal to 1, and the alpha channel storage is destroyed.

Drawables with Unequal Depths

Compositing drawables with unequal depths is undefined.