Preface - Adobe

2.3
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Page Description Languages
consist of one memory cell per pixel. Some implementations use other representations,
such as display lists. The Adobe imaging model has been carefully designed not
to depend on any particular representation of raster memory.
For each graphical element that is to appear on the page, the scan converter sets
the values of the corresponding pixels. When the interpretation of the page description
is complete, the pixel values in memory represent the appearance of the
page. At this point, a raster output process can make this representation visible
on a printed page or a display.
Scan-converting a graphical shape, such as a rectangle or a circle, involves determining
which device pixels lie “inside” the shape and setting their values appropriately
(for example, setting them to black). Because the edges of a shape do not
always fall precisely on the boundaries between pixels, some policy is required for
deciding which pixels along the edges are considered to be “inside.” Scanconverting
a text character is conceptually the same as scan-converting an arbitrary
graphical shape; however, characters are much more sensitive to legibility
requirements and must meet more rigid objective and subjective measures of
quality.
Rendering grayscale elements on a bilevel device—one whose pixels can only be
black or white—is accomplished by a technique known as halftoning. The array
of pixels is divided into small clusters according to some pattern (called the
halftone screen). Within each cluster, some pixels are set to black and some to
white in proportion to the level of gray desired at that place on the page. When
viewed from a sufficient distance, the individual dots become unnoticeable and
the result is a shade of gray. This enables a bilevel raster output device to reproduce
shades of gray and to approximate natural images, such as photographs.
Some color devices use a similar technique.
2.3 Page Description Languages
Theoretically, an application program could describe any page as a full-page pixel
array. But this would be unsatisfactory, because the description would be bulky,
the pixel array would be device-dependent, and memory requirements would be
beyond the capacity of many personal computers.
A page description language should enable applications to produce files that are
relatively compact for storage and transmission, and independent of any particular
output device.