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Soft filling algorithms change the color of an anti-aliased region, maintaining the anti-aliasing of the region. The two published algorithms for soft filling work only if the foreground region is anti-aliased against a black background. This paper presents three new algorithms. The first fills against a region consisting of any two distinct colors, and is faster than the published algorithms on a pixel-by-pixel basis for an RGB frame buffer; the second fills against a region composed of three distinct colors; and the third fills against a region composed of four distinct colors. As the power of the algorithms increases, so do the number of assumptions they make, and the computational cost.
Fishkin et al. (Sun,) studied this question.