Normally a display system produces an analog image in the form of
a rectangular array of display pixels. The brightness of each display pixel is
controlled by the gray level of the corresponding pixel in the digital image.
However, the primary function of the display is to allow the human observer to
understand and interpret the image content. This introduces a subjective elem-
ent, and it is helpful to match the display process to the characteristics of the
human eye. For example, the human eye has considerable acuity in discrimin-
ating fine detail (high-spatial-frequency information), but is not particularly
sensitive to low-frequency (slowly varying) image information [8]. Some images
may be more easily understood if they are displayed indirectly, ...