Xerox 96 NPS manual Device-dependent operators, Gray shading-setscreen and setcolorscreen

ADJUSTING PRINT QUALITY AND SPEED

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.

Note: Depending on the emitter used, color-related operators may be used even for monochrome jobs. On DocuPrint NPS, color jobs undergo a color to monochrome mapping process. Therefore, if these operators are included in the PostScript jobs, they can affect the output.

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.

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).

XEROX DOCUPRINT 96/4635/180 NPS TROUBLESHOOTING GUIDE 7-11