POSTSCRIPT
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. 
XEROX DOCUPRINT NPS GUIDE TO USING PAGE DESCRIPTION LANGUAGES |
