CHAPTER 7
556
Transparency
When preblended image data is used in transparency blending and compositing
computations, the results are the same as if the original, unblended image data
were used and no matte color were specified. In particular, the inputs to the blend
function are the original color values. To derive
c
from
c′,
the application may
sometimes need to invert the formula shown above. If the resulting
c
value lies
outside the range of color component values for the image color space, the results
are unpredictable.
The preblending computation is done in the color space specified by the parent
image’s
ColorSpace
entry. This is independent of the group color space into which
the image may be painted. If a color conversion is required, inversion of the preb-
lending must precede the color conversion. If the image color space is an
Indexed
space (see “Indexed Color Spaces” on page 262), the color values in the color table
(not the index values themselves) are preblended.
7.5.5 Transparency Group XObjects
A transparency group is represented in PDF as a special type of group XObject
(see Section 4.9.2, “Group XObjects”) called a
transparency group XObject.
A
group XObject is in turn a type of form XObject, distinguished by the presence of
a
Group
entry in its form dictionary (see Section 4.9.1, “Form Dictionaries”). The
value of this entry is a subsidiary
group attributes dictionary
defining the proper-
ties of the group. The format and meaning of the dictionary’s contents are deter-
mined by its
group subtype,
which is specified by the dictionary’s
S
entry. The
entries for a transparency group (subtype
Transparency
) are shown in Table 7.13.
Note:
A page object (see “Page Objects” on page 144) may also have a
Group
entry,
whose value is a group attributes dictionary specifying the attributes of the page
group (see Section 7.3.6, “Page Group”). Some of the dictionary entries are inter-
preted slightly differently for a page group than for a transparency group XObject;
see their descriptions in the table for details.
TABLE 7.13 Additional entries specific to a transparency group attributes dictionary
KEY
TYPE
VALUE
S
name
(Required)
The
group subtype,
which identifies the type of group whose at-
tributes this dictionary describes; must be
Transparency
for a transparency
group.