SECTION 3.4
109
File Structure
TABLE 3.16 Entries in a cross-reference stream
TYPE
FIELD
DESCRIPTION
0
1
The type of this entry, which must be 0. Type 0 entries define the
linked list of free objects (corresponding to
f
entries in a cross-
reference table).
The object number of the next free object.
The generation number to use if this object number is used again.
The type of this entry, which must be 1. Type 1 entries define
objects that are in use but are not compressed (corresponding to
n
entries in a cross-reference table).
The byte offset of the object, starting from the beginning of the
file.
The generation number of the object. Default value: 0.
The type of this entry, which must be 2. Type 2 entries define
compressed objects.
The object number of the object stream in which this object is
stored. (The generation number of the object stream is implicitly
0.)
The index of this object within the object stream.
2
3
1
1
2
3
2
1
2
3
Like any stream, a cross-reference stream is an indirect object. Therefore, an
entry for it must exist in either a cross-reference stream (usually itself) or in a
cross-reference table (in hybrid-reference files; see
Compatibility with Applications That Do Not Support PDF 1.5
Applications that do not support PDF 1.5 cannot access objects that are
referenced by cross-reference streams. If a file uses cross-reference streams
exclusively, it cannot be opened by such applications.
However, it is possible to construct a file called a
hybrid-reference
file that is
readable by a PDF 1.4 consumer. Such a file contains objects referenced by
standard cross-reference tables in addition to objects in object streams that are
referenced by cross-reference streams.