Previous Next

                                        479
SECTION 6.1                                          CIE-Based Color to Device Color



optional, full three-dimensional color lookup table. The color fidelity of the out-
put depends on having these parameters properly set, usually by a method that
includes some form of calibration. The colors that a device can produce are char-
acterized by a device profile, which is usually specified by an ICC profile associat-
ed with the device (and entirely separate from the profile that is specified in an
ICCBased color space).

Note: PDF has no equivalent of the PostScript color rendering dictionary. The
means by which a device profile is associated with a consumer application’s output
device are implementation-dependent and cannot be specified in a PDF file. Typi-
cally, this is done through a color management system (CMS) that is provided by the
operating system. Beginning with PDF 1.4, a PDF document can also specify one or
more output intents providing possible profiles that might be used to process the
document (see Section 10.10.4, “Output Intents”).

Conversion from a CIE-based color value to a device color value requires two
main operations:

1. Adjust the CIE-based color value according to a CIE-based gamut mapping
   function. A gamut is a subset of all possible colors in some color space. A page
   description has a source gamut consisting of all the colors it uses. An output
   device has a device gamut consisting of all the colors it can reproduce. This
   step transforms colors from the source gamut to the device gamut in a way that
   attempts to preserve color appearance, visual contrast, or some other explicitly
   specified rendering intent (see “Rendering Intents” on page 260).
2. Generate a corresponding device color value according to a CIE-based color
   mapping function. For a given CIE-based color value, this function computes a
   color value in the device’s native color space.

The CIE-based gamut and color mapping functions are applied only to color
values presented in a CIE-based color space. By definition, color values in device
color spaces directly control the device color components (though this can be al-
tered by the DefaultGray, DefaultRGB, and DefaultCMYK color space resources;
see “Default Color Spaces” on page 257).

The source gamut is specified by a page description when it selects a CIE-based
color space. This specification is device-independent. The corresponding proper-
ties of the output device are given in the device profile associated with the device.
The gamut mapping and color mapping functions are part of the implementation
of the consumer application.

Previous Next