Graphics Programs Reference
In-Depth Information
Table 3-1. Whitespace characters
Character code
Meaning
0
Null
9
Tab
10
Line feed
12
Form feed
13
Carriage return
32
Space
PDF files can use <CR>, <LF>, or a <CR><LF> sequence to end a line. Note, however,
that changing the line endings en masse (for example, in a text editor) will likely corrupt
the file, since it will alter any line ending sequences that happen to occur in the midst
of compressed binary data sections.
Objects
PDF supports five basic objects:
• Integers and real numbers, such as 42 and 3.1415 .
• Strings, which are enclosed in brackets, and come in a variety of encodings. For
example (The Quick Brown Fox) .
• Names, which are used for keys in dictionaries, and innumerable other purposes.
They are introduced with a / , for example /Blue .
• Boolean values, denoted by the keywords true and false .
• The null object, denoted by the keyword null .
and three compound objects:
• Arrays, which contain an ordered collection of other objects such as [1 0 0 0] .
• Dictionaries, which consist of an unordered collection of pairs, mapping names to
objects. For example, <</Contents 4 0 R /Resources 5 0 R>> , which maps /Con
tents to the indirect reference 4 0 R and /Resources to the indirect reference 5 0 R .
• Streams, which hold binary data, together with a dictionary describing attributes
of the data such as its length and any compression parameters. Streams are used
to store images, fonts and so on.
and a way of linking objects together:
• The indirect reference, which forms a link from one object to another.
A PDF file consists of a graph of objects, with indirect references forming the links
between them. The object graph for Example 3-1 is shown in Figure 3-1 .
 
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