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Optimizing print quality
Troubleshooting Guide 5-3
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
If either of the above procedures is successful, the transfer
functions return the same color levels that are passed to them
without modifying or mapping the level. On printers which do not
allow the overwriting of the transfer function, the
settransfer
operator does not affect the mapping of gray values between the
output of the print device and specified levels of color.
NOTE:
Settransfer
actually sets the transfer functions for all
four color components (red, green, blue, and gray) to the same
value. The
setcolortransfer
operator sets the transfer functions
individually.