CHAPTER 4
194
Graphics
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Marked-content operators
associate higher-level logical information with ob-
jects in the content stream. This information does not affect the rendered ap-
pearance of the content (although it may determine if the content should be
presented at all; see Section 4.10, “Optional Content”); it is useful to applica-
tions that use PDF for document interchange. Marked content is described in
This chapter presents general information about device-independent graphics in
PDF: how a PDF content stream describes the abstract appearance of a page.
Rendering—the
device-dependent part of graphics—is covered in Chapter 6. The
concepts and their implementation.
4.1 Graphics Objects
As discussed in Section 3.7.1, “Content Streams,” the data in a content stream is
interpreted as a sequence of
operators
and their
operands,
expressed as basic data
objects according to standard PDF syntax. A content stream can describe the
appearance of a page, or it can be treated as a graphical element in certain other
contexts.
The operands and operators are written sequentially using postfix notation.
Although this notation resembles the sequential execution model of the Post-
Script language, a PDF content stream is not a program to be interpreted; rather,
it is a static description of a sequence of
graphics objects.
There are specific rules,
described below, for writing the operands and operators that describe a graphics
object.
PDF provides five types of graphics objects:
•
A
path object
is an arbitrary shape made up of straight lines, rectangles, and
cubic Bézier curves. A path may intersect itself and may have disconnected
sections and holes. A path object ends with one or more painting operators that
specify whether the path is stroked, filled, used as a clipping boundary, or some
combination of these operations.
•
A
text object
consists of one or more character strings that identify sequences of
glyphs to be painted. Like a path, text can be stroked, filled, or used as a clip-
ping boundary.