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Preface - Adobe

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2.4<br />

15<br />

Using the PostScript Language<br />

gram to be executed, rather than data to be consumed. Dynamic page<br />

description languages contain elements of programming languages, such as<br />

procedures, variables, and control constructs.<br />

The PostScript language design is dynamic. The language includes a set of primitive<br />

graphics operators that can be combined to describe the appearance of any<br />

printed or displayed page. It has variables and allows arbitrary computations<br />

while interpreting the page description. It has a rich set of programminglanguage<br />

control structures for combining its elements.<br />

2.4 Using the PostScript Language<br />

It is important to understand the PostScript interpreter and how it interacts with<br />

applications using it.<br />

2.4.1 The Interpreter<br />

The PostScript interpreter controls the actions of the output device according to<br />

the instructions provided in a PostScript program generated by an application.<br />

The interpreter executes the program and produces output on a printer, display,<br />

or other raster device.<br />

There are three ways the PostScript interpreter and the application interact<br />

(Figure 2.1 illustrates these scenarios):<br />

• In the conventional output-only printing model, the application creates a page<br />

description—a self-contained PostScript language description of a document.<br />

The page description can be sent to the PostScript interpreter immediately or<br />

stored for transmission at some other time (via an intermediate print manager<br />

or spooler, for example). The interpreter consumes a sequence of page descriptions<br />

as “print jobs” and produces the requested output. The output device is<br />

typically a printer, but it can be a preview window on a workstation’s display.<br />

The PostScript interpreter is often implemented on a dedicated processor that<br />

has direct control over the raster output device.<br />

• In the integrated display model, an application interacts with the PostScript<br />

interpreter controlling a display or windowing system. Instead of a one-way<br />

transmission of a page description, a two-way interactive session takes place<br />

between the application and the interpreter. In response to user actions, the

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