CHAPTER 6
486
Rendering
duce device-dependent effects, a page description that is intended to be device-
independent should not alter them.
Note:
When the current color space is
DeviceGray
and the output device’s native
color space is
DeviceCMYK
, the interpreter uses only the gray transfer function. The
normal conversion from
DeviceGray
to
DeviceCMYK
produces 0.0 for the cyan,
magenta, and yellow components. These components are not passed through their
respective transfer functions but are rendered directly, producing output containing
no colored inks. This special case exists for compatibility with existing applications
that use a transfer function to obtain special effects on monochrome devices, and
applies only to colors specified in the
DeviceGray
color space.
See Section 7.6.4, “Rendering Parameters and Transparency,” and in particular,
of transfer functions in the transparent imaging model.
6.4 Halftones
Halftoning
is a process by which continuous-tone colors are approximated on an
output device that can achieve only a limited number of discrete colors. Colors
that the device cannot produce directly are simulated by using patterns of pixels
in the colors available. Perhaps the most familiar example is the rendering of gray
tones with black and white pixels, as in a newspaper photograph.
Some output devices can reproduce continuous-tone colors directly. Halftoning is
not required for such devices; after gamma correction by the transfer functions,
the color components are transmitted directly to the device. On devices that do
require halftoning, it occurs after all color components have been transformed by
the applicable transfer functions. The input to the halftone function consists of
continuous-tone, gamma-corrected color components in the device’s native color
space. Its output consists of pixels in colors the device can reproduce.
PDF provides a high degree of control over details of the halftoning process. For
example, in color printing, independent halftone screens can be specified for each
of several colorants. When rendering on low-resolution displays, fine control
over halftone patterns is needed to achieve the best approximations of gray levels
or colors and to minimize visual artifacts.
Note:
Remember that everything pertaining to halftones is, by definition, device-
dependent. In general, when a PDF document provides its own halftone specifica-