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Chapter 7. Transparency > Specifying Transparency in PDF

7.5. Specifying Transparency in PDF

The preceding sections have presented the transparent imaging model at an abstract level, with little mention of its representation in PDF. This section describes the facilities available for specifying transparency in PDF 1.4.

7.5.1. Specifying Source and Backdrop Colors

Single graphics objects, as defined in Section 4.1, “Graphics Objects,” are treated as elementary objects for transparency compositing purposes (subject to special treatment for text objects, as described in Section 5.2.7, “Text Knockout”). That is, all of a given object is considered to be one element of a transparency stack. Portions of an object are not composited with one another, even if they are described in a way that would seem to cause overlaps (such as a self-intersecting path, combined fill and stroke of a path, or a shading pattern containing an overlap or fold-over). An object's source color Cs, used in the color compositing formula, is specified in the same way as in the opaque imaging model: by means of the current color in the graphics state or the source samples in an image. The backdrop color Cb is the result of previous painting operations.


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