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CHAPTER 7                                                                 Transparency



of the parent XObject’s image dictionary (see Section 4.8.4, “Image Dictionaries”;
see also implementation note 73 in Appendix H). Entries in the subsidiary image
dictionary for such a soft-mask image have the same format and meaning as in
that of an ordinary image XObject (as described in Table 4.39 on page 340), sub-
ject to the restrictions listed in Table 7.11. This type of image dictionary can also
optionally contain an additional entry, Matte, discussed below.

When an image is accompanied by a soft-mask image, it is sometimes advanta-
geous for the image data to be preblended with some background color, called the
matte color. Each image sample represents a weighted average of the original
source color and the matte color, using the corresponding mask sample as the
weighting factor. (This is a generalization of a technique commonly called pre-
multiplied alpha.)

If the image data is preblended, the matte color must be specified by a Matte
entry in the soft-mask image dictionary (see Table 7.12). The preblending com-
putation, performed independently for each component, is
c' = m + α × ( c – m )

where
  c′ is the value to be provided in the image source data
  c is the original image component value
  m is the matte color component value
  α is the corresponding mask sample

Note: This computation uses actual color component values, with the effects of the
Filter and Decode transformations already performed. The computation is the same
whether the color space is additive or subtractive.

          TABLE 7.11 Restrictions on the entries in a soft-mask image dictionary
KEY                        RESTRICTION

Type                       If present, must be XObject.

Subtype                    Must be Image.

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