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Desktop Color Primer
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 (color example 11a). 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 Photoshop
and Painter.
The amount of data found in a raster image depends on its resolution and bit depth.
The resolution of a raster describes the compactness 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 raster images require only one bit of information per pixel.
Grayscale images require 8 bits per pixel. For photographic quality color, 24 bits of
RGB color information are required per pixel, yielding 256 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 source raster. If the raster 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 objects are defined mathematically as lines or curves between
points—hence the term “vector” (color example 11b). Picture elements can have solid,
gradient, or patterned color fills. Vector artwork is created in illustration and drawing
applications, such as 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 and resolution
without danger of pixels becoming visible in printed output.
