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DESKTOP COLOR PRIMER 74

Raster images and vector graphics

Two broad categories of artwork can be printed from a personal computer to a color printer:

raster images and vector graphics.

A raster image, also referred to as a bitmap, is composed of a grid of pixels, each assigned a

particular color value (as illustrated in example a in the following figure). 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 Corel 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 graphics, picture objects are defined mathematically as lines or curves between

points—hence the term “vector” (see example b). 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. You can scale it to any size and resolution without

danger of pixels becoming visible in printed output.

a b

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KYOCERA 6550ci Manuale Pagina 74