SECTION 7.5
551
Specifying Transparency in PDF
group and apply the mask to the group as a whole. These considerations also apply
to the current alpha constant (see the next section).
Constant Shape and Opacity
The
current alpha constant
parameter in the graphics state (see Section 4.3,
other painting operations—to be used for the constant shape
(f
k
)
or constant
opacity
(q
k
)
component in the color compositing formulas. This parameter can
be thought of as analogous to the current color used when painting elementary
objects. (Note, however, that the nonstroking alpha constant is also applied when
painting a transparency group’s results onto its backdrop; see also implementa-
The stroking and nonstroking alpha constants are set, respectively, by the
CA
and
ca
entries in a graphics state parameter dictionary (see Section 4.3.4, “Graphics
source flag in the graphics state determines whether the alpha constants are inter-
preted as shape values (
true
) or opacity values (
false
).
Note:
The note at the end of “Mask Shape and Opacity,” above, applies to the cur-
rent alpha constant parameter as well as the current soft mask.
7.5.4 Specifying Soft Masks
As noted under “Mask Shape and Opacity” on page 550, soft masks for use in
compositing computations can be specified in one of the following ways:
•
As a soft-mask dictionary in the current soft mask parameter of the graphics
state; see “Soft-Mask Dictionaries,” below, for more details.
•
As a soft-mask image associated with a sampled image; see “Soft-Mask Images”
•
(In PDF 1.5) as a mask channel embedded in JPEG2000 encoded data; see Sec-
SMaskInData
entry of Table 4.39 for
more details.