Xerox 96 NPS manual Settransfer, or Setcolortransfer, Smooth curves-setflat

ADJUSTING PRINT QUALITY AND SPEED

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.

Smooth curves—setflat

PostScript masters that use the PostScript operator setflat produce inconsistent output across different PostScript printers. setflat controls curve rendering smoothness. PostScript curve operators use cubic Bezier control points to define the curve shape. These curves can be rendered from straight line segments. Normally the line segments are so short that the curve appears smooth. The setflat operator indirectly controls the length of the straight line segments. As stated in the PostScript Language Reference Manual, second edition, “If the flatness parameter is large enough to cause visible straight line segments to appear, the result is unpredictable. setflat sets a graphics state parameter whose effect is device- dependent. It should not be used in a page description that is intended to be device-independent.”

Invisible strokes—0 setlinewidth

PostScript masters that use the PostScript operator setlinewidth are not consistent across different PostScript printers. setlinewidth controls the width of a stroked line. When setlinewidth is executed with an input of zero, it produces a line that is one pixel wide. On devices whose dot size is small, single-pixel-width lines may be invisible.

Scan conversion—fill, eofill, and stroke

Scan conversion algorithms are implementation-dependent, so different PostScript printers paint (“turn on”) different dots, resulting in output differences. For filled circles, PostScript printers differ in the dots they paint at the edge of the circle; thus, some printers produce slightly larger circles than others. For large circles this is not noticeable, but for small circles, it is.

Sections 2.2 and 6.5 of the PostScript Language Reference Manual, second edition, discuss scan conversion. Section 6.5 states that scan conversion details are not part of the PostScript standard.

7-12 XEROX DOCUPRINT 96/4635/180 NPS TROUBLESHOOTING GUIDE