CHAPTER 6
6
Rendering
The Adobe imaging model separates
graphics
(the specification of shapes and col-
ors) from
rendering
(controlling a raster output device). Figures 4.12 and 4.13 on
pages 238 and 239 illustrate this division. Chapter 4 describes the facilities for
specifying the appearance of pages in a device-independent way. This chapter de-
scribes the facilities for controlling how shapes and colors are rendered on the
raster output device. All of the facilities discussed here depend on the specific
characteristics of the output device. PDF documents that are intended to be de-
vice-independent should limit themselves to the general graphics facilities de-
scribed in Chapter 4.
Nearly all of the rendering facilities that are under the control of a PDF document
pertain to the reproduction of color. Colors are rendered by a multiple-step pro-
cess outlined below. (Depending on the current color space and on the character-
istics of the device, it is not always necessary to perform every step.)
1. If a color has been specified in a CIE-based color space (see Section 4.5.4,
native color
space
of the raster output device (also called its
process color model).
2. If a color has been specified in a device color space that is inappropriate for the
output device (for example,
RGB
color with a
CMYK
or grayscale device), a
color conversion function
is invoked.
3. The device color values are now mapped through
transfer functions,
one for
each color component. The transfer functions compensate for peculiarities of
the output device, such as nonlinear gray-level response. This step is some-
times called
gamma correction.
4. If the device cannot reproduce continuous tones, but only certain discrete
colors such as black and white pixels, a
halftone function
is invoked, which
approximates the desired colors by means of patterns of pixels.
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