Halftone postprocessing for improved rendition of highlights and shadows

Many binary halftoning algorithms tend to render extreme tones (i.e., very light or very dark tones) with objectionable dot distributions, To alleviat e this artifact we introduce a halftone postprocessing algorithm called the Springs algorithm. The objective of Springs is to rearrange minority pixel s in affected regions for a smoother, more attractive rendition. In this pa per, we describe the Springs algorithm, and we show results which demonstra te its effectiveness. The heart of this algorithm is a simple dot-rearrange ment heuristic which results in a more isotropic dot distribution. The appr oach is to treat any well-isolated dot as if it were connected to neighbori ng dots by springs, and to move it to a location where the energy in the sp rings is a minimum. Applied to the whole image, this could degrade halftone appearance. However, Springs only moves dots in selected regions of the im age. Pixels that are not minority pixels are not moved at all. Moreover, do t rearrangement is disabled on and around detected edges, since it could ot herwise render those edges soft and diffuse. (C) 2000 SPIE and IS&T. [S1017 -9909(00)00102-1].