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Working with Color
Raster images and vector images
Two broad categories of artwork can be printed from a personal computer
to a color printer: raster and vector images.
A raster image, also referred to as a bitmap, is composed of a grid of
pixels, each assigned a particular color value. The grid, when sufficiently
enlarged, resembles a mosaic made from square tiles. Examples of raster
images include scans and images created in painting or pixel-editing
applications, such as Adobe 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
information 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
resolution 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
sometimes called "pixelation".
In vector images, picture elements are defined mathematically as lines or
curves between points—hence the term "vector". Picture elements can
have solid, gradient, or patterned color fills. Vector artwork is created in
illustration and drawing applications such as Adobe Illustrator and
CorelDRAW. Page layout 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
without danger of pixels becoming visible in printed output.
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