jbm@eos.arc.nasa.gov (Jeffrey Mulligan) (02/21/91)
bahl@Gauss.dec.com (P. Bahl (Victor)) writes: >I am looking for references on multibit dithering (Ordered and/or Error >Diffusion). There is a very simple method for the special case where the grey scale is to be quantized uniformly (equal steps): Say you want to quantize to N levels, and that the input is normalized to a min of 0 and a max of N-1. First quantize the image to N-1 levels by rounding down, resulting in an integer image ranging from 0 to N-2. (To insure that no pixels get quantized to N-1 you have to tweak the normalization so that the max is slightly less than N-1). Now subtract the quantized image from the original, which results in a non-negative error image ranging from 0 to 1. Apply your favorite binary dithering algorithm to this error image. Now add the dithered error image to the quantized (truncated) image, with the "on" pixels in the dithered error image "promoting" selected pixels in the quantized image to the next level. I discussed this briefly in an SPIE paper from 1990 (of which I have reprints). There are also other, more complex algorithms which can deal with nonuniform quantization and arbitrary color palettes, for which I cannot supply references off-hand. For arbitrary color palettes, this usually boils down to finding the "best" color; there are two halves to the problem: the search (maybe it can be restricted), and deciding what metric to use to decide which of two palette colors is "closer" to the target color. General formulas for characterizing perceptual color differences is an active area of psychophysical research. -- Jeff Mulligan (jbm@eos.arc.nasa.gov) NASA/Ames Research Ctr., Mail Stop 262-2, Moffett Field CA, 94035 (415) 604-3745