SECTION 7.6
565
Color Space and Rendering Issues
The opaque imaging model also allows process color components to be addressed
individually, as if they were spot colors. For instance, it is possible to specify a
Separation
color space named
Cyan
, which paints just the cyan component on a
CMYK
output device. However, this capability is very difficult to extend to trans-
parency groups. In general, the color components in a group are not the process
colorants themselves, but are converted to process colorants only after the com-
pletion of all color compositing computations for the group (and perhaps some of
its parent groups as well). For instance, if the group’s color space is
ICCBased
, the
group has no
Cyan
component to be painted. Consequently, treating a process
color component as if it were a spot color is permitted only within a group that
inherits the native color space of the output device (or is implicitly converted to
DeviceCMYK
, as discussed in “Implicit Conversion of CIE-Based Color Spaces”
sults in conversion of the requested spot color to its alternate color space.
7.6.3 Overprinting and Transparency
In the opaque imaging model, overprinting is controlled by two parameters of the
graphics state: the
overprint parameter
and the
overprint mode
(see Section 4.5.6,
rants to be marked, as determined by the current color space and current color in
the graphics state. The remaining colorants are either erased or left unchanged,
depending on whether the overprint parameter is
false
or
true
. When the current
color space is
DeviceCMYK
, the overprint mode parameter additionally enables
this selective marking of colorants to be applied to individual color components
according to whether the component value is zero or nonzero.
Because this model of overprinting deals directly with the painting of device
colorants, independently of the color space in which source colors have been
specified, it is highly device-dependent and primarily addresses production
needs rather than design intent. Overprinting is usually reserved for opaque colo-
rants or for very dark colors, such as black. It is also invoked during late-stage
production operations such as trapping (see Section 10.10.5, “Trapping Sup-
Consequently, it is best to think of transparency as taking place in appearance
space, but overprinting of device colorants in device space. This means that colo-
rant overprint decisions should be made at output time, based on the actual re-
sultant colorants of any transparency compositing operation. On the other hand,
effects similar to overprinting can be achieved in a device-independent manner
by taking advantage of blend modes, as described in the next section.