HP L 5 manual Color Appearance Matching, Color Lookup Tables

Color Appearance Matching

Color appearance matching goes beyond true color matching by including adjustments for the dynamic ranges of the devices, so the user's intent is maintained. For example, the white areas of a page shown on a monitor display screen would be printed as white on a printed page because the user specified white, even though the screen cannot duplicate a white that truly matches white paper. Although the printed color does not exactly match screen color, color appearance does match, which is what users usually want. To maximize user satisfaction, the PCL language uses appearance matching when rendering device-independent color.

Color Lookup Tables

Color lookup tables, which provide additional control of the printed output, are transformations that map input data into a new output color range based upon point-by-point conversions.

Overhead transparencies provide one example of a good use for color lookup tables. Let’s say a page is printed on plain paper and it matches the user’s expectations. When printing the same document on overhead transparency film, the resulting image looks unsaturated and flat. To compensate, the user can send a color lookup table to increase color saturation without changing composition (for example, using the CIE L*a*b* color space to increase the a* and b* parameters in equal amounts).

Color lookup tables can also be used to adjust data from a Kodak CD-ROM, which uses the Photo YCC device- independent color space. The gamma correction table is complex and cannot be described by the traditional logarithmic expression. However, since the data can be mapped into new data values via tables, the user can provide a gamma correction table that essentially describes the complex correction factors.

Color lookup tables can be used to “neutral-balance” an image. For example, an underwater photograph produces a severe bluish cast when printed. The user can eliminate that cast from the image by providing a color lookup table that subtracts some color portion from each of the primaries.

A-12 Color Printing Overview (Color LaserJet, 5, 5M, DeskJet) EN