CHAPTER 7
572
Transparency
SOURCE COLOR SPACE
AFFECTED COLOR
COMPONENT OF
GROUP COLOR SPACE
VALUE OF BLEND FUNCTION
B
(c
b
,
c
s
) EXPRESSED AS TINT
OP FALSE
OP TRUE, OPM 0
OP TRUE, OPM 1
A group (not an
elementary object)
All color
components
c
s
c
s
c
s
Color component values are represented in these tables as subtractive tint values
because overprinting is typically applied to subtractive colorants such as inks
rather than to additive ones such as phosphors on a display screen. The Compati-
bleOverprint blend mode is therefore described as if it took subtractive argu-
ments and returned subtractive results. In reality, however, CompatibleOverprint
(like all blend modes) treats color components as additive values; subtractive
components must be complemented before and after application of the blend
function.
Note an important difference between the two tables. In Table 7.14, the process
color components being discussed are the actual device colorants—the color
components of the output device’s native color space (
DeviceGray
,
DeviceRGB
, or
DeviceCMYK
). In Table 7.15, the process color components are those of the
group’s color space, which is not necessarily the same as that of the output device
(and can even be something like
CalRGB
or
ICCBased
). For this reason, the pro-
cess color components of the group color space cannot be treated as if they were
spot colors in a
Separation
or
DeviceN
color space (see Section 7.6.2, “Spot
overprinting and erasing rules arises only within a transparency group (including
the page group, if its color space is different from the native color space of the
output device). There is no difference in the treatment of spot color components.
ject that is a transparency group rather than an elementary object (fill, stroke,
text, image, or shading). As stated in Section 7.6.2, “Spot Colors and Transparen-
Color components that were not explicitly painted by any object in the group
have an additive color value of 1.0 (subtractive tint 0.0). Since no information is
retained about which components were actually painted within the group, com-
patible overprinting is not possible in this case; the CompatibleOverprint blend
mode reverts to
Normal
, with no consideration of the overprint and overprint
mode parameters. (A transparency-aware application can choose a more suitable
blend mode, such as
Darken
, to produce an effect similar to overprinting.)