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                                        483
SECTION 6.2                                   Conversions among Device Color Spaces



In PDF 1.2, the black-generation and undercolor-removal functions are defined
as PDF function dictionaries (see Section 3.9, “Functions”) that are parameters in
the graphics state. They are specified as the values of the BG and UCR (or BG2 and
UCR2) entries in a graphics state parameter dictionary (see Table 4.8 on page
220). Each function is called with a single numeric operand and is expected to
return a single numeric result.

The input of both the black-generation and undercolor-removal functions is k,
the minimum of the intermediate c, m, and y values that have been computed by
subtracting the original red, green, and blue components from 1.0. Nominally, k is
the amount of black that can be removed from the cyan, magenta, and yellow
components and substituted as a separate black component.

The black-generation function computes the black component as a function of
the nominal k value. It can simply return its k operand unchanged, or it can re-
turn a larger value for extra black, a smaller value for less black, or 0.0 for no
black at all.

The undercolor-removal function computes the amount to subtract from each of
the intermediate c, m, and y values to produce the final cyan, magenta, and yellow
components. It can simply return its k operand unchanged, or it can return 0.0
(so that no color is removed), some fraction of the black amount, or even a nega-
tive amount, thereby adding to the total amount of colorant.

The final component values that result after applying black generation and un-
dercolor removal are expected to be in the range 0.0 to 1.0. If a value falls outside
this range, the nearest valid value is substituted automatically without error indi-
cation. This substitution is indicated explicitly by the min and max operations in
the formulas above.

The correct choice of black-generation and undercolor-removal functions de-
pends on the characteristics of the output device—for example, how inks mix.
Each device is configured with default values that are appropriate for that device.

See Section 7.6.4, “Rendering Parameters and Transparency,” and in particular,
“Rendering Intent and Color Conversions” on page 574, for further discussion of
the role of black-generation and undercolor-removal functions in the transparent
imaging model.

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