ABSTRACT
Halftoning via blue-noise dither arrays is a process whereby a continuoustone image is thresholded on a pixel-by-pixel basis with a dither array or mask. A pixel of the resulting binary image is set to one if that pixel of the continuous-tone image is greater than or equal to the corresponding pixel of the dither array; otherwise, the pixel is set to zero. The blue-noise dither array derives its name from the fact that, given a continuous-tone monochrome image of constant gray-level g, the resulting dither pattern has blue-noise characteristics appropriate to g. A popular scheme for building blue-noise dither arrays is Robert Ulichney void-and-cluster (VAC) technique, which iteratively swaps black and white pixels according to a measure of local white pixel density. A significant feature of VAC is that by performing the low-pass filtering by means of circular convolution, the resulting dither pattern has wrap-around characteristics such that tiling a pattern end-to-end creates no apparent discontinuities in texture.
