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                                         552
CHAPTER 7                                                               Transparency



Soft-Mask Dictionaries

The most common way of defining a soft mask is with a soft-mask dictionary
specified as the current soft mask in the graphics state (see Section 4.3, “Graphics
State”). Table 7.10 shows the contents of this type of dictionary. (See implementa-
tion note 72 in Appendix H.)

The mask values are derived from those of a transparency group, using one of the
two methods described in Sections 7.4.1, “Deriving a Soft Mask from Group
Alpha,” and 7.4.2, “Deriving a Soft Mask from Group Luminosity.” The group is
defined by a transparency group XObject (see Section 7.5.5, “Transparency
Group XObjects”) designated by the G entry in the soft-mask dictionary. The S
(subtype) entry specifies which of the two derivation methods to use:

• If the subtype is Alpha, the transparency group XObject G is evaluated to com-
  pute a group alpha only. The colors of the constituent objects are ignored and
  the color compositing computations are not performed. The transfer function
  TR is then applied to the computed group alpha to produce the mask values.
  Outside the bounding box of the transparency group, the mask value is the re-
  sult of applying the transfer function to the input value 0.0.
• If the subtype is Luminosity, the transparency group XObject G is composited
  with a fully opaque backdrop whose color is everywhere defined by the soft-
  mask dictionary’s BC entry. The computed result color is then converted to a
  single-component luminosity value, and the transfer function TR is applied to
  this luminosity to produce the mask values. Outside the transparency group’s
  bounding box, the mask value is derived by transforming the BC color to lumi-
  nosity and applying the transfer function to the result.

The mask’s coordinate system is defined by concatenating the transformation
matrix specified by the Matrix entry in the transparency group’s form dictio-
nary (see Section 4.9.1, “Form Dictionaries”) with the current transformation
matrix at the moment the soft mask is established in the graphics state with the
gs operator.

Note: In a transparency group XObject that defines a soft mask, spot color compo-
nents are never available, even if they are available in the group or page on which
the soft mask is used. If the group XObject’s content stream specifies a Separation or
DeviceN color space that uses spot color components, the alternate color space is
substituted (see “Separation Color Spaces” on page 264 and “DeviceN Color Spaces”
on page 268).

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