CHAPTER 2
34
Overview
that allows it to be searched, edited, or extracted for reuse elsewhere. PDF is par-
ticularly well suited for representing a document as it moves through successive
stages of a prepress production workflow.
2.1 Imaging Model
At the heart of PDF is its ability to describe the appearance of sophisticated
graphics and typography. This ability is achieved through the use of the
Adobe
imaging model,
the same high-level, device-independent representation used in
the PostScript page description language.
Although application programs could theoretically describe any page as a full-
resolution pixel array, the resulting file would be bulky, device-dependent, and
impractical for high-resolution devices. A high-level imaging model enables
applications to describe the appearance of pages containing text, graphical
shapes, and sampled images in terms of abstract graphical elements rather than
directly in terms of device pixels. Such a description is economical and device-
independent, and can be used to produce high-quality output on a broad range of
printers, displays, and other output devices.
2.1.1 Page Description Languages
Among its other roles, PDF serves as a
page description language,
a language for
describing the graphical appearance of pages with respect to an imaging model.
An application program produces output through a two-stage process:
1. The application generates a device-independent description of the desired out-
put in the page description language.
2. A program controlling a specific output device interprets the description and
renders
it on that device.
The two stages may be executed in different places and at different times. The
page description language serves as an interchange standard for the compact, de-
vice-independent transmission and storage of printable or displayable docu-
ments.