SECTION 4.9
361
Form XObjects
group
attributes dictionary
describing the properties of the group.
As shown in Table 4.46, every group XObject has a
group subtype
(specified by
the
S
entry in the group attributes dictionary) that determines the format and
meaning of the dictionary’s remaining entries. Only one such subtype is currently
defined, a
transparency group XObject
(subtype
Transparency
) representing a
transparency group for use in the transparent imaging model (see Section 7.3,
described in Section 7.5.5, “Transparency Group XObjects.”
TABLE 4.46 Entries common to all group attributes dictionaries
KEY
TYPE
VALUE
Type
name
name
(Optional)
The type of PDF object that this dictionary describes; if present, must
be
Group
for a group attributes dictionary.
(Required)
The
group subtype,
which identifies the type of group whose at-
tributes this dictionary describes and determines the format and meaning of the
dictionary’s remaining entries. The only group subtype defined in PDF 1.4 is
Transparency
; see Section 7.5.5, “Transparency Group XObjects,” for the re-
maining contents of this type of dictionary. Other group subtypes may be added
in the future.
S
4.9.3 Reference XObjects
Reference XObjects (PDF 1.4)
enable one PDF document to import content from
another. The document in which the reference occurs is called the
containing
document;
the one whose content is being imported is the
target document.
The
target document may reside in a file external to the containing document or may
be included within it as an embedded file stream (see Section 3.10.3, “Embedded
The reference XObject in the containing document is a form XObject containing
the optional
Ref
entry in its form dictionary, as described below. This form XOb-
ject serves as a
proxy
that can be displayed or printed in place of the imported
content. The proxy might consist of a low-resolution image of the imported con-
tent, a piece of descriptive text referring to it, a gray box to be displayed in its
place, or any other similar placeholder. PDF consumers that do not recognize the
Ref
entry simply display or print the proxy as an ordinary form XObject (see im-