CHAPTER 7
550
Transparency
All elementary objects have an intrinsic opacity
q
j
of 1.0 everywhere. Any desired
opacity less than 1.0 must be applied by means of an opacity mask or constant, as
described in the following sections.
Mask Shape and Opacity
At most one mask input—called a
soft mask,
or
alpha mask—can
be provided to
any PDF compositing operation. The mask can serve as a source of either shape
(f
m
)
or opacity
(q
m
)
values, depending on the setting of the
alpha source
parame-
ter in the graphics state (see Section 4.3, “Graphics State”). This is a boolean flag,
set with the
AIS
(“alpha is shape”) entry in a graphics state parameter dictionary
true
if the soft mask
contains shape values,
false
for opacity.
The soft mask can be specified in one of the following ways:
•
The
current soft mask
parameter in the graphics state, set with the
SMask
entry
in a graphics state parameter dictionary, contains a
soft-mask dictionary
(see
name
None
may be specified in place of a soft-mask dictionary, denoting the
absence of a soft mask. In this case, the mask shape or opacity is implicitly 1.0
everywhere. (See implementation note 72 in Appendix H.)
•
An image XObject can contain its own
soft-mask image
in the form of a subsid-
iary image XObject in the
SMask
entry of the image dictionary (see Section
or key mask specified by the image dictionary’s
Mask
entry. Either form of
mask in the image dictionary overrides the current soft mask in the graphics
state. (See implementation note 73 in Appendix H.)
•
An image XObject that has a
JPXDecode
filter as its data source can specify an
SMaskInData
entry, indicating that the soft mask is embedded in the data
stream (see Section 3.3.8, “JPXDecode Filter”).
Note:
The current soft mask in the graphics state is intended to be used to clip only a
single object at a time (either an elementary object or a transparency group). If a
soft mask is applied when painting two or more overlapping objects, the effect of the
mask multiplies with itself in the area of overlap (except in a knockout group), pro-
ducing a result shape or opacity that is probably not what is intended. To apply a
soft mask to multiple objects, it is usually best to define the objects as a transparency