Many image processing applications are intended to produce images that are to be viewed by human observers (as opposed to, say, automated industrial inspection.) It is therefore important to understand the characteristics and limitations of the human visual system--to understand the "receiver" of the 2D signals. At the outset it is important to realize that 1) the human visual system is not well understood, 2) no objective measure exists for judging the quality of an image that corresponds to human assessment of image quality, and, 3) the "typical" human observer does not exist. Nevertheless, research in perceptual psychology has provided some important insights into the visual system. See, for example, Stockham .