CHAPTER 4
262
Graphics
NAME
DESCRIPTION
Perceptual
Colors are represented in a manner that provides a pleasing per-
ceptual appearance. To preserve color relationships, both in-
gamut and out-of-gamut colors are generally modified from
their precise colorimetric values. A typical use might be for
scanned images.
4.5.5 Special Color Spaces
Special color spaces add features or properties to an underlying color space.
There are four special color space families:
Pattern
,
Indexed
,
Separation
, and
DeviceN
.
Pattern Color Spaces
A
Pattern
color space
(PDF 1.2)
enables a PDF content stream to paint an area
with a
pattern
rather than a single color. The pattern may be either a
tiling pattern
(type 1) or a
shading pattern
(type 2). Section 4.6, “Patterns,” discusses patterns in
detail.
Indexed Color Spaces
An
Indexed
color space allows a PDF content stream to use small integers as indi-
ces into a
color map
or
color table
of arbitrary colors in some other space. A PDF
consumer application treats each sample value as an index into the color table
and uses the color value it finds there. This technique can considerably reduce the
amount of data required to represent a sampled image—for example, by using
8-bit index values as samples instead of 24-bit
RGB
color values.
An
Indexed
color space is defined by a four-element array:
[ /Indexed
base hival lookup
]
The first element is the color space family name
Indexed
. The remaining ele-
ments are parameters that an
Indexed
color space requires; their meanings are
discussed below. Setting the current stroking or nonstroking color space to an
Indexed
color space initializes the corresponding current color to 0.