Raster Images And Vector Images - Xerox DocuColor DocuColor 4 User Manual

6.4

Raster images and vector images

Two broad categories of artwork can be printed from a personal computer to
a color printer/copier: raster and vector images.
raster image, also referred to as a bitmap, is composed of a grid of pixels,
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each assigned a particular color value. The grid, when sufficiently enlarged,
resembles a mosaic made from square tiles. Examples of raster images in-
clude scans and images created in painting or pixel-editing applications, such
as Photoshop and Painter.
The amount of information found in a raster depends on its resolution and
bit depth. The resolution of a raster describes the density of the pixels and is
specified in pixels per inch (ppi). The bit depth is the number of bits of infor-
mation assigned to each pixel. Black and white rasters require only one bit
of information per pixel. For photographic quality color, 24 bits of RGB color
information are required per pixel, yielding 256 separate levels of red, green,
and blue. For CMYK images, 32 bits per pixel are required.
When printing raster artwork, the quality of the output depends on the reso-
lution of the raster. If the raster's resolution is too low, individual pixels
become visible in the printed output as small squares. This effect is some-
times called "pixelation."
In vector images, picture elements are defined mathematically as lines or
curves between points-hence the term "vector." Picture elements can have
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solid, gradient, or patterned color fills. Vector artwork is created in illustra-
tion and drawing applications such as Illustrator and CorelDRAW. Page lay-
out applications such as QuarkXPress also allow you to create simple vector
artwork with their drawing tools. PostScript fonts are vector-based as well.
Vector artwork is resolution-independent; it can be scaled to any size with-
out danger of pixels becoming visible in printed output.
Print the files and examine the output. You will likely begin to see a marked
deterioration in output quality at resolutions below 200 ppi, while above
200 ppi the improvement may be very subtle.
Raster images prepared for offset printing may need to be at higher resolu-
tion 6.6.3
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