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