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CHAPTER 7                                                              Transparency



Spot colors are inherently device-dependent and are not always available. In the
opaque imaging model, each use of a spot color component in a Separation or
DeviceN color space is accompanied by an alternate color space and a tint transfor-
mation function for mapping tint values into that space. This enables the color to
be approximated with process colorants when the corresponding spot colorant is
not available on the device.

Spot colors can be accommodated straightforwardly in the transparent imaging
model (except for issues relating to overprinting, discussed in Section 7.6.3,
“Overprinting and Transparency”). When an object is painted transparently with
a spot color component that is available in the output device, that color is com-
posited with the corresponding spot color component of the backdrop, indepen-
dently of the compositing that is performed for process colors. A spot color
retains its own identity; it is not subject to conversion to or from the color space
of the enclosing transparency group or page. If the object is an element of a trans-
parency group, one of two things can happen:

• The group maintains a separate color value for each spot color component,
  independently of the group’s color space. In effect, the spot color passes directly
  through the group hierarchy to the device, with no color conversions per-
  formed. However, it is still subject to blending and compositing with other ob-
  jects that use the same spot color.
• The spot color is converted to its alternate color space. The resulting color is
  then subject to the usual compositing rules for process colors. In particular,
  spot colors are never available in a transparency group XObject that is used to
  define a soft mask; the alternate color space is always substituted in that case.

Only a single shape value and opacity value are maintained at each point in the
computed group results; they apply to both process and spot color components.
In effect, every object is considered to paint every existing color component, both
process and spot. Where no value has been explicitly specified for a given com-
ponent in a given object, an additive value of 1.0 (or a subtractive tint value of
0.0) is assumed. For instance, when painting an object with a color specified in a
DeviceCMYK or ICCBased color space, the process color components are painted
as specified and the spot color components are painted with an additive value of
1.0. Likewise, when painting an object with a color specified in a Separation color
space, the named spot color is painted as specified and all other components
(both process colors and other spot colors) are painted with an additive value of
1.0. The consequences of this are discussed in Section 7.6.3, “Overprinting and
Transparency.”

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