Floyd-Steinberg will almost always give the best looking results; however, looking good is not always what you want. For instance, thresholding can be used in a pipeline with the ppmconvol tool, for tasks like edge and peak detection. And clustered-dot dithering gives a newspaper-ish look, a useful special effect.
The -value flag alters the thresholding value for Floyd-Steinberg and simple threshholding. It should be a real number between 0 and 1. Above 0.5 means darker images; below 0.5 means lighter.
All flags can be abbreviated to their shortest unique prefix.
Note that there is no pbmtopgm converter, because any pgm program can read pbm files automagically.
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