CHAPTER 6
480
Rendering
6.2 Conversions among Device Color Spaces
Each raster output device has a
native color space,
which typically is one of the
standard device color spaces (
DeviceGray
,
DeviceRGB
, or
DeviceCMYK
). In other
words, most devices support reproduction of colors according to a grayscale
(monochrome),
RGB
(red-green-blue), or
CMYK
(cyan-magenta-yellow-black)
model. If the device supports continuous-tone output, reproduction occurs di-
rectly. Otherwise, it is accomplished by means of halftoning.
A device’s native color space is also called its
process color model.
Process colors
are ones that are produced by combinations of one or more standard
process
colorants.
Colors specified in any device or CIE-based color space are rendered as
process colors. (A device can also support additional
spot colorants,
which can be
painted only by means of
Separation
or
DeviceN
color spaces. They are not in-
volved in the rendering of device or CIE-based color spaces, nor are they subject
to the conversions described below.)
Note:
Some devices provide a native color space that is not one of the three named
above but consists of a different combination of colorants. In that case, conversion
from the standard device color spaces to the device’s native color space is performed
by device-dependent means.
Knowing the native color space and other output capabilities of the device, the
consumer application can automatically convert the color values specified in a
document to those appropriate for the device’s native color space. For example, if
a document specifies colors in the
DeviceRGB
color space but the device supports
grayscale (such as a monochrome display) or
CMYK
(such as a color printer), the
consumer application performs the necessary conversions. If the document spec-
ifies colors directly in the device’s native color space, no conversions are neces-
sary.
The algorithms used to convert among device color spaces are very simple. As
perceived by a human viewer, the conversions produce only crude approxima-
tions of the original colors. More sophisticated control over color conversion can
be achieved by means of CIE-based color specification and rendering. Addition-
ally, device color spaces can be remapped into CIE-based color spaces (see