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5-2 Troubleshooting Guide
Device-dependent operators
Some PostScript operators result in device-dependent PostScript
files. “Device dependence” means that devices having different
output characteristics may produce different-looking output even
though they use the same software. This section documents the
most serious examples of device dependence. The
PostScript
Language Reference Manual,
second edition, lists all of the
graphics state operators that are considered device-dependent.
Gray shading—setscreen and setcolorscreen
The PostScript operators setscreen and setcolorscreen control
the PostScript halftone software that simulates various intensity
levels of gray and color inks. Since each PostScript product may
have a different default halftone definition, the halftone patterns
and gray scale range may differ also. Because setscreen is
inherently device dependent (especially when used for pattern
fills), PostScript masters look different on almost every different
model of PostScript printer.
The operators are also used to perform pattern fills, since
PostScript Level 1 does not have a pattern fill operator.
(PostScript Level 2 has pattern fill operators.) As stated in the
PostScript Language Reference Manual
, first edition,
“Remember that everything pertaining to halftones is, by
definition, device-dependent. In general, when an application
provides its own halftone specifications, it sacrifices portability.
Associated with every device is a default halftone definition that
is appropriate for most applications.”
In addition to the device-dependent nature of halftoning, a spot
function is used by
setscreen
and
setcolorscreen
to determine
the order of pixel darkening in the halftone cell. When two pixels
have the same spot function value, their ordering values are
determined arbitrarily. Duplicate spot function values are very
common. In fact, almost all the various default spot functions
generate duplicate values. Therefore, different PostScript
implementations yield different halftone patterns, resulting in
visual differences in the output.