CHAPTER 7
548
Transparency
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 de-
scribed 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
C
s
, 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
C
b
is the result of previous painting operations.
7.5.2 Specifying Blending Color Space and Blend Mode
The blending color space is an attribute of the transparency group within which
an object is painted; its specification is described in Section 7.5.5, “Transparency
page group
(see Section 7.3.6, “Page Group”), with a color space attribute of its own. If not
otherwise specified, the page group’s color space is inherited from the native color
space of the output device.
The blend mode
B
(C
b
,
C
s
) is determined by the
current blend mode
parameter in
the graphics state (see Section 4.3, “Graphics State”), which is specified by the
BM
entry in a graphics state parameter dictionary (Section 4.3.4, “Graphics State Pa-
standard blend modes listed in Tables 7.2 and 7.3 on pages 520 and 524, or an ar-
ray of such names. In the latter case, the application should use the first blend
mode in the array that it recognizes (or
Normal
if it recognizes none of them).
Therefore, new blend modes can be introduced in the future, and applications
that do not recognize them have reasonable fallback behavior. (See implementa-
Note:
The current blend mode always applies to process color components but only
sometimes to spot colorants; see “Blend Modes and Overprinting” on page 566 for
details.