SECTION 7.6
569
Color Space and Rendering Issues
Plate 20 shows the effects of all four possible combinations of blending and over-
printing, using the
Screen
blend mode in the
DeviceCMYK
color space. The label
“overprint enabled” means that the overprint parameter in the graphics state is
true
and the overprint mode is 1. In the upper half of the figure, a light green oval
is painted opaquely (opacity = 1.0) over a backdrop shading from pure yellow to
pure magenta. In the lower half, the same object is painted with transparency
(opacity = 0.5).
Special Path-Painting Considerations
The overprinting considerations discussed above also affect those path-painting
operations that combine filling and stroking a path in a single operation. These
include the
B
,
B*
,
b
, and
b*
operators (see Section 4.4.2, “Path-Painting Opera-
fill and stroke are treated as a single graphics object, as if they were enclosed in a
transparency group. This implicit group is established and used as follows:
•
If overprinting is enabled (the overprint parameter in the graphics state is
true
)
and the current stroking and nonstroking alpha constants are equal, a non-
isolated, non-knockout transparency group is established. Within the group,
the fill and stroke are performed with an alpha value of 1.0 but with the Com-
patibleOverprint blend mode. The group results are then composited with the
backdrop, using the originally specified alpha and blend mode.
•
In all other cases, a non-isolated knockout group is established. Within the
group, the fill and stroke are performed with their respective prevailing alpha
constants and the prevailing blend mode. The group results are then composit-
ed with the backdrop, using an alpha value of 1.0 and the
Normal
blend mode.
Note that in the case of showing text with the combined filling and stroking text
rendering modes, this behavior is independent of the text knockout parameter in
the graphics state (see Section 5.2.7, “Text Knockout”).
The purpose of these rules is to avoid having a non-opaque stroke composite with
the result of the fill in the region of overlap, which would produce a double bor-
der effect that is usually undesirable. The special case that applies when the over-
print parameter is
true
is for backward compatibility with the overprinting
behavior of the opaque imaging model. If a desired effect cannot be achieved with
a combined filling and stroking operator or text rendering mode, it can be