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    Thanks! Since a colored image has RBG three channels, what does/can the threshold apply to in general? Commented Jan 9, 2014 at 23:00
  • @Tim It probably depends on the software, but I would expect the threshold (unless you specify per channel or for a specific channel, see e.g. ImageMagick's convert's -channel option) to be applied to some sort of "value" of the pixel, which is computed from all channels. That's the reason why I said you might want to convert to grayscale first for more predictable results. (Also see my edit.) Commented Jan 10, 2014 at 8:17
  • Thanks! Does there exist some documentation for what the threshold apply to in the command shown in your post? Commented Jan 10, 2014 at 18:29
  • @Tim Not really. I expected convert to take either a percentage of the maximum value (which should have been 256 per channel) or a specific value, but I could only get a useful result when specifying a percentage. When you do it with a graphical tool, including GIMP, you'll generally have a histogram that shows the tonal distribution of the image; that will be a great aid in picking the proper value. Using only the command line, unless you have a specific reason to do so, is probably more trouble than it's worth, really. Commented Jan 10, 2014 at 18:31
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    As a side note, there exist other thresholding methods that don't have to rely on a hard-coded threshold level. For example, ImageMagick includes -lat which performs a local adaptive threshold, taking into account the surrounding pixels. Commented Jan 15, 2014 at 22:37