SECTION 2.1
37
Imaging Model
individual pixels makes it possible to generate printed or displayed output that
can include text, arbitrary graphical shapes, and reproductions of sampled images.
The
resolution
of a raster output device measures the number of pixels per unit of
distance along the two linear dimensions. Resolution is typically—but not neces-
sarily—the same horizontally and vertically. Manufacturers’ decisions on device
technology and price/performance trade-offs create characteristic ranges of reso-
lution:
•
Computer displays have relatively low resolution, typically 75 to 110 pixels per
inch.
•
Dot-matrix printers generally range from 100 to 250 pixels per inch.
•
Ink-jet and laser-scanned xerographic printing technologies achieve medium-
level resolutions of 300 to 1400 pixels per inch.
•
Photographic technology permits high resolutions of 2400 pixels per inch or
more.
Higher resolution yields better quality and fidelity of the resulting output but is
achieved at greater cost. As the technology improves and computing costs de-
crease, products evolve to higher resolutions.
2.1.4 Scan Conversion
An abstract graphical element (such as a line, a circle, a character glyph, or a
sampled image) is rendered on a raster output device by a process known as
scan
conversion.
Given a mathematical description of the graphical element, this pro-
cess determines which pixels to adjust and what values to assign to those pixels to
achieve the most faithful rendition possible at the available device resolution.
The pixels on a page can be represented by a two-dimensional array of pixel
values in computer memory. For an output device whose pixels can only be black
or white, a single bit suffices to represent each pixel. For a device that can repro-
duce gray levels or colors, multiple bits per pixel are required.
Note:
Although the ultimate representation of a printed or displayed page is logically
a complete array of pixels, its actual representation in computer memory need not
consist of one memory cell per pixel. Some implementations use other representa-
tions, such as display lists. The Adobe imaging model has been carefully designed
not to depend on any particular representation of raster memory.