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 alleviate this artifact, we introduce a halftone postprocessing algorithm called the Springs algorithm. The objective of Springs is to rearrange minority pixels in affected regions for a smoother, more attractive rendition. In this paper, we describe the Springs algorithm, and we show results which demonstrate its effectiveness. The heart of this algorithm is a simple dot-rearrangement heuristic which results in a more isotropic dot distribution. The approach is to treat any well-isolated dot as if it were connected to neighboring dots by springs, and to move it to a location where the energy in the springs is a minimum. Applied to the whole image, this could degrade halftone appearance. However, Springs only moves dots in selected regions of the image. Pixels that are not minority pixels are not moved at all. Moreover, dot rearrangement is disabled on and around detected edges, since it could otherwise render those edges soft and diffuse. © 2000 SPIE and IS&T. [S1017-9909(00)00102-1]