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SECTION 4.5                                                                Color Spaces



Given a formula for converting any combination of black and gold tints to cali-
brated RGB, a 2-in, 3-out type 4 (PostScript calculator) function could be used for
the tint transformation. Alternatively, a type 0 (sampled) function could be used,
but this would require a large number of sample points to represent the function
accurately; for example, sampling each input variable for 256 tint values between
0.0 and 1.0 would require 256 2 = 65,536 samples. But since the DeviceN color
space is being used as the base of an Indexed color space, there are actually only
256 possible combinations of black and gold tint values. A more compact way to
represent this information is to put the alternate color values directly into the
lookup table alongside the DeviceN color values, as in Example 4.19.

Example 4.19

  10 0 obj                                              % Color space
     [ /Indexed
           [ /DeviceN
                 [ /Black /Gold /None /None /None ]
                 [ /CalRGB
                         << /WhitePoint [ 1.0 1.0 1.0 ]
                            /Gamma [ 2.2 2.2 2.2 ]
                         >>
                 ]
                 20 0 R                                 % Tint transformation function
           ]
           255
           … Lookup table …
     ]
  endobj

In this example, each entry in the lookup table has five components: two for the
black and gold colorants and three more (specified as None) for the equivalent
CalRGB color components. If the black and gold colorants are available on the
output device, the None components are ignored; if black and gold are not
available, the tint transformation function is used to convert a five-component
color into a three-component equivalent in the alternate CalRGB color space.
But because, by construction, the third, fourth, and fifth components are the
CalRGB components, the tint transformation function can merely discard the
first two components and return the last three. This can be easily expressed
with a type 4 (PostScript calculator) function, as shown in Example 4.20.

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