SECTION 7.5
557
Specifying Transparency in PDF
KEY
TYPE
VALUE
CS
name or array
(Sometimes required, as discussed below)
The group color space, which is used
for the following purposes:
•
As the color space into which colors are converted when painted into the
group
•
As the blending color space in which objects are composited within the group
(see Section 7.2.3, “Blending Color Space”)
•
As the color space of the group as a whole when it in turn is painted as an ob-
ject onto its backdrop
The group color space may be any device or CIE-based color space that treats its
components as independent additive or subtractive values in the range 0.0 to 1.0,
subject to the restrictions described in Section 7.2.3, “Blending Color Space.”
These restrictions exclude
Lab
and lightness-chromaticity
ICCBased
color spac-
es, as well as the special color spaces
Pattern
,
Indexed
,
Separation
, and
DeviceN
.
Device color spaces are subject to remapping according to the
DefaultGray
,
DefaultRGB
, and
DefaultCMYK
entries in the
ColorSpace
subdictionary of the
current resource dictionary (see “Default Color Spaces” on page 257).
Ordinarily, the
CS
entry is allowed only for isolated transparency groups (those
for which
I
, below, is
true
), and even then it is optional. However, this entry is re-
quired in the group attributes dictionary for any transparency group XObject
that has no parent group or page from which to inherit—in particular, one that
is the value of the
G
entry in a soft-mask dictionary of subtype
Luminosity
(see
In addition, it is always permissible to specify
CS
in the group attributes dictio-
nary associated with a page object, even if
I
is
false
or absent. In the normal case
in which the page is imposed directly on the output medium, the page group is
effectively isolated regardless of the
I
value, and the specified
CS
value is there-
fore honored. But if the page is in turn used as an element of some other page
and if the group is non-isolated,
CS
is ignored and the color space is inherited
from the actual backdrop with which the page is composited (see Section 7.3.6,
Default value: the color space of the parent group or page into which this trans-
parency group is painted. (The parent’s color space in turn can be either explicit-
ly specified or inherited.)
Note:
For a transparency group XObject used as an annotation appearance (see
page on which the annotation appears.