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EN Raster Graphics 6-1
6
Raster Graphics
Introduction
A raster image is a made up of a series of discrete picture elements—
pixels. Pictures such as those in newspapers, television, and
documents from Hewlett-Packard printers are examples of raster
images. In comparison, pictures drawn with lines and geometric
objects such as circles, rectangles, and polygons are termed vector
graphic images. While some pictures could be produced using either
vector or raster graphics, raster graphics is best suited for printing
photographic images.
A raster image is invariably rectangular and divided into a gridwork of
pixels. Each pixel represents a small area of the image. In
monochrome images each pixel represents a black or white dot. In
color images each pixel is a colored dot. Therefore, the most
important characteristics of an image are its:
• Image width
• Image height
• Image resolution (the number of dots or pixels per inch)
An image is also divided into rows, where a row is a grouping of all
the pixels in a horizontal strip of the image, the width of the image and
one pixel high.
As noted before, a pixel represents a small area of the image. The
size of the area depends on the resolution of the image. The pixel
area for a 600 dot per inch (dpi) image is a square 1/600th of a inch
on a side. Courser resolutions cover more picture area per pixel. For
example, a 75 dpi pixel contains 0.0002 square inches, a 600 dpi
pixel contains 0.000003 square inches. Some printing and scanning
devices specify a resolution in the horizontal direction and another
resolution in the vertical direction, 300 by 600 dpi, for example.
However, PCL raster only supports identical resolutions in the
horizontal and vertical directions, and therefore, PCL 5 raster
graphics pixels are squares.