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E-22 A-61122/A-61124 March 1999
Computer-generated images
Computer-generated images (printed by dot-matrix or laser
printers) are discrete in both tone and space. Discrete in tone
means the image consists of pixels on a grid.
The number of bits-per-pixel determine the number of gray levels
available. The spacing between points on the grid determine the
resolution of the image. Fewer bits-per-pixel or large grid spacing
reduce computer-generated image quality.
Scanning computer-generated images can degrade image quality
through aliasing and moiré patterns. (The “Evaluation criteria”
section describes aliasing and moiré).
Photographic images
Photographic images are continuous in both tone and space. A
continuous tone image can assume all possible shades of gray.
Image elements are not restricted to specific points of a grid
pattern.
Image quality is usually measured by gray density and resolution.
Optics and illumination can degrade photographic image quality.
Scanning photographic images can result in degraded image
quality due to the translation of infinite gray shades to a finite
number of gray levels generated by a scanner.