CHAPTER 4
236
Graphics
•
Color rendering.
The application reproduces colors on the raster output device
by a multiple-step process that includes some combination of color conversion,
gamma correction, halftoning, and scan conversion. Some aspects of this pro-
cess use information that is specified in PDF. However, unlike the facilities for
color specification, the color-rendering facilities are device-dependent and or-
dinarily should not be included in a page description.
Figures 4.12 and 4.13 on pages 238 and 239 illustrate the division between PDF’s
(device-independent) color specification and (device-dependent) color-render-
ing facilities. This section describes the color specification features, covering
everything that most PDF documents need to specify colors. The facilities for
controlling color rendering are described in Chapter 6; a PDF document should
use these facilities only to configure or calibrate an output device or to achieve
special device-dependent effects.
4.5.1 Color Values
As described in Section 4.4.2, “Path-Painting Operators,” marks placed on the
page by operators such as
f
and
S
have a color that is determined by the
current
color
parameter of the graphics state. A color value consists of one or more
color
components,
which are usually numbers. For example, a gray level can be speci-
fied by a single number ranging from 0.0 (black) to 1.0 (white). Full color values
can be specified in any of several ways; a common method uses three numeric
values to specify red, green, and blue components.
Color values are interpreted according to the
current color space,
another pa-
rameter of the graphics state. A PDF content stream first selects a color space by
invoking the
CS
operator (for the stroking color) or the
cs
operator (for the non-
stroking color). It then selects color values within that color space with the
SC
operator (stroking) or the
sc
operator (nonstroking). There are also conve-
nience operators—
G
,
g
,
RG
,
rg
,
K
, and
k
—that select both a color space and a
color value within it in a single step. Table 4.24 on page 287 lists all the color-
setting operators.
Sampled images (see Section 4.8, “Images”) specify the color values of individual
samples with respect to a color space designated by the image object itself. While
these values are independent of the current color space and color parameters in
the graphics state, all later stages of color processing treat them in exactly the
same way as color values specified with the
SC
or
sc
operator.