Gray Shading-Settransfer And Transfer Function - Xerox DocuPrint 155 Troubleshooting Manual

Optimizing print quality
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.
Gray shading—settransfer and transfer function
The PostScript operators settransfer and setcolortransfer are
used to change the procedure for mapping perceived color
values in the output of the print device into specified color values
in the PostScript master. PostScript devices use an internal
transfer array or procedure that takes into account the printing
characteristics of their marking engines in order to produce
levels of color.
Transfer functions are called by the setcmykcolor, setbsbcolor,
setrgbcolor, and setgray operator before processing of the
desired color level that is specified in the master. The transfer
function takes a value from the stack and leaves another value
on the stack for each color component (red, green, blue, and
gray).
The PostScript language provides a scheme for overwriting the
internal transfer function of a printing device by providing the
settransfer and setcolortransfer operators. However, some
Xerox PostScript devices do not allow you to overwrite their
internal transfer function.
Typically, a master tries to replace the internal transfer function
with a null transfer function such as:
{} settransfer, or
{} {} {} {} setcolortransfer
Troubleshooting Guide 6-5