SECTION 7.5
553
Specifying Transparency in PDF
TABLE 7.10 Entries in a soft-mask dictionary
KEY
TYPE
VALUE
Type
name
name
(Optional)
The type of PDF object that this dictionary describes; if present,
must be
Mask
for a soft-mask dictionary.
(Required)
A subtype specifying the method to be used in deriving the mask
values from the transparency group specified by the
G
entry:
Alpha
S
Use the group’s computed alpha, disregarding its color (see
Convert the group’s computed color to a single-component
luminosity value (see Section 7.4.2, “Deriving a Soft Mask
Luminosity
G
stream
(Required)
A transparency group XObject (see Section 7.5.5, “Transparency
ing the mask. If the subtype
S
is
Luminosity
, the group attributes dictionary
must contain a
CS
entry defining the color space in which the compositing
computation is to be performed.
(Optional)
An array of component values specifying the color to be used as
the backdrop against which to composite the transparency group XObject
G
.
This entry is consulted only if the subtype
S
is
Luminosity
. The array consists
of
n
numbers, where
n
is the number of components in the color space speci-
fied by the
CS
entry in the group attributes dictionary (see Section 7.5.5,
value, representing black.
(Optional)
A function object (see Section 3.9, “Functions”) specifying the
transfer function to be used in deriving the mask values. The function ac-
cepts one input, the computed group alpha or luminosity (depending on the
value of the subtype
S
), and returns one output, the resulting mask value.
Both the input and output must be in the range 0.0 to 1.0; if the computed
output falls outside this range, it is forced to the nearest valid value. The
name
Identity
may be specified in place of a function object to designate the
identity function. Default value:
Identity
.
BC
array
TR
function or name
Soft-Mask Images
The second way to define a soft mask is by associating a
soft-mask image
with an
image XObject. This is a subsidiary image XObject specified in the
SMask
entry