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SECTION 7.2                                         Basic Compositing Computations



Of the PDF color spaces described in Section 4.5, “Color Spaces,” the following
are supported as blending color spaces:

• DeviceGray
• DeviceRGB
• DeviceCMYK
• CalGray
• CalRGB
• ICCBased color spaces equivalent to those above (including calibrated CMYK)
The Lab space and ICCBased spaces that represent lightness and chromaticity sep-
arately (such as L*a*b*, L*u*v*, and HSV) are not allowed as blending color spac-
es because the compositing computations in such spaces do not give meaningful
results when applied separately to each component. In addition, an ICCBased
space used as a blending color space must be bidirectional; that is, the ICC profile
must contain both AToB and BToA transformations.

The blending color space is consulted only for process colors. Although blending
can also be done on individual spot colors specified in a Separation or DeviceN
color space, such colors are never converted to a blending color space (except in
the case where they first revert to their alternate color space, as described under
“Separation Color Spaces” on page 264 and “DeviceN Color Spaces” on page
268). Instead, the specified color components are blended individually with the
corresponding components of the backdrop.

The blend functions for the various blend modes assume that the range for each
color component is 0.0 to 1.0 and that the color space is additive. The former
condition is true for all of the allowed blending color spaces, but the latter condi-
tion is not true. In particular, the DeviceCMYK, Separation, and DeviceN spaces
are subtractive. When performing blending operations in subtractive color spac-
es, it is assumed that the color component values are complemented (subtracted
from 1.0) before the blend function is applied and that the results of the function
are then complemented back before being used. This adjustment makes the ef-
fects of the various blend modes numerically consistent across all color spaces.
However, the actual visual effect produced by a given blend mode still depends
on the color space. Blending in a device color space produces device-dependent
results, whereas in a CIE-based space it produces results that are consistent
across all devices. See Section 7.6, “Color Space and Rendering Issues,” for addi-
tional details concerning color spaces.

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