CHAPTER 4
284
Graphics
As in the preceding examples, an
Indexed
color space based on a
DeviceN
space is
used to paint the grayscale image shown on the left in the plate with four colo-
rants: black and three PANTONE spot colors. The alternate color space is a sim-
ple calibrated
RGB.
Thus, the
DeviceN
color space has seven components: the
four desired colorants plus the three components of the alternate space. The ex-
ample shows the image XObject (see Section 4.8.4, “Image Dictionaries”) repre-
senting the quadtone image, followed by the color space used to interpret the
image data. (See implementation note 49 in Appendix H.)
4.5.6 Overprint Control
The graphics state contains an
overprint parameter,
controlled by the
OP
and
op
entries in a graphics state parameter dictionary. Overprint control is useful main-
ly on devices that produce true physical separations, but it is available on some
composite devices as well. Although the operation of this parameter is device-
dependent, it is described here rather than in the chapter on color rendering,
because it pertains to an aspect of painting in device color spaces that is impor-
tant to many applications.
Any painting operation marks some specific set of device colorants, depending
on the color space in which the painting takes place. In a
Separation
or
DeviceN
color space, the colorants to be marked are specified explicitly; in a device or
CIE-based color space, they are implied by the process color model of the output
device (see Chapter 6). The overprint parameter is a boolean flag that determines
how painting operations affect colorants other than those explicitly or implicitly
specified by the current color space.
If the overprint parameter is
false
(the default value), painting a color in any color
space causes the corresponding areas of unspecified colorants to be erased (paint-
ed with a tint value of 0.0). The effect is that the color at any position on the page
is whatever was painted there last, which is consistent with the normal painting
behavior of the opaque imaging model.
If the overprint parameter is
true
and the output device supports overprinting, no
such erasing actions are performed; anything previously painted in other colo-
rants is left undisturbed. Consequently, the color at a given position on the page
may be a combined result of several painting operations in different colorants.
The effect produced by such overprinting is device-dependent and is not defined
by the PDF language.