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CHAPTER 7 Transparency
7.2.4 Blend Mode
In principle, the blend function B (Cb , Cs ), used in the compositing formula to
customize the blending operation, could be any function of the backdrop and
source colors that yields another color, Cr, for the result. PDF defines a standard
set of named blend functions, or blend modes, listed in Tables 7.2 and 7.3. Plates
18 and 19 illustrate the resulting visual effects for RGB and CMYK colors, respec-
tively.
A blend mode is termed separable if each component of the result color is com-
pletely determined by the corresponding components of the constituent back-
drop and source colors—that is, if the blend mode function B is applied
separately to each set of corresponding components:
cr = B ( cb , cs )
where the lowercase variables cr, cb , and cs denote corresponding components of
the colors Cr, Cb , and Cs , expressed in additive form. (Theoretically, a blend
mode could have a different function for each color component and still be sepa-
rable; however, none of the standard PDF blend modes have this property.) A
separable blend mode can be used with any color space, since it applies indepen-
dently to any number of components. Only separable blend modes can be used
for blending spot colors.
Table 7.2 lists the standard separable blend modes available in PDF.
TABLE 7.2 Standard separable blend modes
NAME RESULT
Normal Selects the source color, ignoring the backdrop:
B ( cb , cs ) = cs
Multiply Multiplies the backdrop and source color values:
B ( cb , cs ) = cb × cs
The result color is always at least as dark as either of the two constituent colors. Multiply-
ing any color with black produces black; multiplying with white leaves the original color
unchanged. Painting successive overlapping objects with a color other than black or white
produces progressively darker colors.
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