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POSTSCRIPT
2-22 XEROX DOCUPRINT NPS GUIDE TO USING PAGE DESCRIPTION LANGUAGES
Rendering of color and gray values: transfer function
PostScript devices approximate source color values by a sampling
and halftoning technique. These values can be modified by changing
the output transfer function. The transfer function permits the
correction of color (including gray) values to compensate for
nonlinear color (and gray) level response in both an output device
and the human eye.
The color levels presented to the setcmykcolor, sethsbcolor,
setrgbcolor, and setgray operators and appearing as sampled data
are not used directly, but are mapped through the current transfer
functions. The resulting output consists of combinations of black and
white on the DocuPrint monochrome printer, and black, white, and
highlight color on the DocuPrint highlight color printers.
Note: The internal default transfer function in DocuPrint cannot be
bypassed. Currently, there is an internal table that is used for
converting input requests for a color value to a different number that
is used to set the output device color level.
On the DocuPrint monochrome printer, PostScript masters that rely
on the null transfer function (null transfer function is {} settransfer)
do not overwrite the internal transfer function.
Furthermore, the operator currenttransfer returns what appears to
be a null transfer function, that is, {}. However, the internal transfer
function is not null. This achieves compatibility, because the
xerography differs between DocuPrint and LaserWriter.
Note: For DocuPrint monochrome printers only, provision is made
for compatibility with pattern generation, as long as the spot function
returns values of 0 or 1.
When a gray value is presented to the setgray operator, DocuPrint
NPS performs the following processes:
1. Computes the index by multiplying the gray value by 255.0 and
rounding the result:
index = Round (gray x 255.0 + 0.5)
2. Sets the device gray value to the value in the transfer array at
the which the index was calculated:
device color = transferarray[index]
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.