Atkinson dithering

Atkinson dithering is an image dithering algorithm designed by Bill Atkinson at Apple Computer and used in the original Macintosh computer. It is a variant of Floyd-Steinberg dithering and works by error diffusion: the image is scanned from top to bottom and left to right, each pixel is quantized to the available tones one at a time, and the residual quantization error is pushed onto neighboring pixels that have not yet been processed. If several pixels have been rounded down, the next pixel becomes more likely to be rounded up, so the average error is reduced. Unlike Floyd-Steinberg dithering, only three quarters of the error is diffused outward, which gives a more localized dither. The cost is lower performance on near-white and near-black areas, but the increase in contrast there may be preferred for some purposes.