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Is there a single word to denote reducing the color palette of an image to two colors: black and white? For instance, navy becomes black and beige turns into white.

I know there is desaturate, but it's too broad, can mean even a slight reduction in color.

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    I was hoping that "to gray scale" would be a verb, but I couldn't find it used as such in any dictionary. You still could use it, I suppose. I rather like the idea of "grayscaling" something. Also black-and-white images mostly have more shades than black and white. EDIT: On second thought, you should ignore this and go with Andrew Leach's answer.
    – Kaiser Octavius
    Jul 9, 2013 at 13:56
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    @KaiserOctavius, I'm suprised no-one has started saying grey-scale. I look forward to monochrome or monotint appearing as verbs too.
    – Brian Hooper
    Jul 9, 2013 at 14:13
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    Except that both monochrome and monotint mean "grey-scale", and also include a similar "red-scale" or "blue-scale" -- graduated tints of a single colour. And they are all already commonly used as verbs.
    – Andrew Leach ♦
    Jul 9, 2013 at 14:20
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    There is decolor or decolorize, and while those mean remove color, they are usually equated to bleaching and contemplate lightening dark colors, not shifting them to black.
    – bib
    Jul 9, 2013 at 15:24
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    This question appears to be off-topic because it requires a specialist term for which the Graphic Design Stack would be more suited.
    – Andrew Leach ♦
    Jul 9, 2013 at 15:25

3 Answers 3

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The proper term for converting an image to black and white is binarize. The process of doing so is called binarization.

From Wiktionary:

binarize (third-person singular simple present binarizes, present participle binarizing, simple past and past participle binarized)

  1. (mathematics) To represent in binary
  2. To convert (an image) to only black and white.

An example usage would be: "The first step is to binarize the image by applying a threshold."

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    Yay! Thanks so much for the precise term and the threshold example!
    – katspaugh
    Jul 9, 2013 at 19:30
  • All photographs are already represented in binary.
    – tchrist ♦
    Jul 10, 2013 at 0:02
  • @tchrist That "binary" has nothing to do with what we are talking about here.
    – Kris
    Jul 10, 2013 at 7:23
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This question might be better asked at GD.SE but there is posterize:

posterize verb
[with object]
print or display (a photograph or other image) using only a small number of different tones:
   posterize the image and view the result

[ODO]

The term is derived from letterpress printing of posters, where only a limited number of solid colours were easily used.

Original and posterized images (using Paint Shop Pro's posterize function):

Queen Elizabeth II Conference Centre, London Posterized image

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  • The inner form and semantics of "posterize" seems even further than "desaturate". The images you provided, though, illustrate gray scale and b&w beautifully. Thanks!
    – katspaugh
    Jul 9, 2013 at 14:08
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    @katspaugh Added explanation: "The term is derived from letterpress printing of posters, where only a limited number of solid colours were easily used." You could have more than just one colour (black) on white.
    – Andrew Leach ♦
    Jul 9, 2013 at 14:14
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    But you can also posterize a color picture, and that results in an image still in color, but with a non-continuous tonal range with only a few (usually primary) colors.
    – bib
    Jul 9, 2013 at 15:12
  • @bib Exactly. A limited number of solid colours. This answer describes either two-colour posterization [for most uses these days] or one-colour posterization [for letterpress printing with real ink].
    – Andrew Leach ♦
    Jul 9, 2013 at 15:14
  • @AndrewLeach But the OP specifically is asking for only black and white as the result [see comments], and the example he offers is already posterized - a two color blue/beige being converted to a two color black/white.
    – bib
    Jul 9, 2013 at 15:21
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I agree with most of the suggestions above and will offer a few more:

  • Adopt the terminology "convert [source] to destination," as insisting on making everything a verb just makes most of the verbs very clumsy
  • threshold could be a synonym for posterizing to monochrome
  • halftone to 1-bit monochrome would cover another example of output in only one colorant (trading off spatial resolution for pixel depth)
    • but then you may want to state what kind of halftone (clustered dot screens with frequency and angle, or stochastic screens, or hybrids)
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  • Thanks, Liudvikas! How would you use threshold in a sentence like "Let's convert this image to x" or "This color x-ed would be black"?
    – katspaugh
    Jul 9, 2013 at 16:38
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    "Let's threshold the grayscale image at the median value" would be shorthand for "Let's select a threshold value T such that half the pixels are lighter than T, half are darker than T, and output an image where pixels are black when input pixels are lighter than the threshold T." That would be comprehensible to someone who knows what Andrew Leach's posterized example is actually doing. In general, "to threshold" is image processing shorthand for "do one thing for input that exceeds some value, another thing for input that doesn't".
    – Liudvikas Bukys
    Jul 9, 2013 at 20:41

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