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SECTION 7.6                                        Color Space and Rendering Issues



Normal or Darken blend modes used for these purposes may not be suitable for
use on those components whose color values are specified. In particular, using
the Darken blend mode for such components would preclude overprinting a dark
color with a lighter one. Moreover, some other blend mode may be specifically
desired for those components.

The PDF graphics state specifies only one current blend mode parameter, which
always applies to process colorants and sometimes to spot colorants as well.
Specifically, only separable, white-preserving blend modes can be used for spot
colors. A blend mode is white-preserving if its blend function B has the property
that B (1.0, 1.0) = 1.0. (Of the standard separable blend modes listed in Table 7.2
on page 520, all except Difference and Exclusion are white-preserving.) If the
specified blend mode is not separable and white-preserving, it applies only to
process color components; the Normal blend mode is substituted for spot colors.
This ensures that when objects accumulate in an isolated transparency group, the
accumulated values for unspecified components remain 1.0 as long as only white-
preserving blend modes are used. The group’s results can then be overprinted us-
ing Darken (or other useful modes) while avoiding unwanted interactions with
components whose values were never specified within the group.


Compatibility with Opaque Overprinting

Because the use of blend modes to achieve effects similar to overprinting does
not make direct use of the overprint control parameters in the graphics state,
such methods are usable only by transparency-aware applications. For compati-
bility with the methods of overprint control used in the opaque imaging model, a
special blend mode, CompatibleOverprint, is provided that consults the over-
print-related graphics state parameters to compute its result. This mode applies
only when painting elementary graphics objects (fills, strokes, text, images, and
shadings). It is never invoked explicitly and is not identified by any PDF name
object; rather, it is implicitly invoked whenever an elementary graphics object is
painted while overprinting is enabled (that is, when the overprint parameter in
the graphics state is true).

Note: Earlier designs of the transparent imaging model included an additional
blend mode named Compatible, which explicitly invoked the CompatibleOverprint
blend mode described here. Because CompatibleOverprint is now invoked implicitly
whenever appropriate, it is never necessary to specify the Compatible blend mode
for use in compositing. It is still recognized as a valid blend mode for the sake of
compatibility but is simply treated as equivalent to Normal.

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