Visual Performance And Image Coding

Sample spacing and quantization levels are usually chosen for digitizing images such that the eye should not see degradations due to either process. Sample spacing is chosen based on the resolution (or high frequency) limit of the eye and quantization is based on perception of low contrast differences at lower frequencies. This process results in about 8 bit/pixel, 20 pixel/mm digitization, but, being based on two different visual limits, the total number of bits is an overestimate of the information perceived by the eye. The visual MTF can be interpreted in terms of perceptible levels as a function of spatial frequency. We show by this interpretation that the total information perceived by the eye is much less than 8 bits times the number of pixels. We consider the classic halftone as an image coding process, yielding 1 bit/ pixel. This approach indicates that halftones approximate the proper distribution of levels as a function of spatial frequency; therefore we have a possible explanation of why halftone images retain most of the visual quality of the original.