SECTION 3.4
101
File Structure
term is used regardless of whether the stream is actually encoded with a
compression filter.)
Any PDF object can appear in an object stream, with the following exceptions:
Stream objects
Objects with a generation number other than zero
A document’s encryption dictionary (see Section 3.5, “Encryption”)
An object representing the value of the
Length
entry in an object stream dictio-
nary
Note:
In addition, in linearized files (see Appendix F, “Linearized PDF”), the docu-
ment catalog, the linearization dictionary, and page objects may not appear in an
object stream.
Indirect references to objects inside object streams use the normal syntax: for
example,
14 0 R
. Access to these objects requires a different way of storing cross-
reference information; see Section 3.4.7, “Cross-Reference Streams.” Although an
application must support PDF 1.5 to use compressed objects, the objects can be
stored in a manner that is compatible with PDF 1.4. Applications that do not
support PDF 1.5 can ignore the objects; see
“Compatibility with Applications That
Do Not Support PDF 1.5” on page 109.
In addition to the standard keys for streams shown in Table 3.4, the stream
dictionary describing an object stream contains the following entries:
TABLE 3.14 Additional entries specific to an object stream dictionary
KEY
TYPE
DESCRIPTION
Type
name
integer
integer
stream
(Required)
The type of PDF object that this dictionary describes; must be
ObjStm
for an object stream.
(Required)
The number of compressed objects in the stream.
(Required)
The byte offset (in the decoded stream) of the first compressed object.
(Optional)
A reference to an object stream, of which the current object stream is
considered an extension. Both streams are considered part of a
collection
of object
streams (see below). A given collection consists of a set of streams whose
Extends
links form a directed acyclic graph.
N
First
Extends
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