How to convert a photo to a black and white image by ImageMagick?

Question

How can I convert a JPEG photo to black and white (not grayscale) image like output of a FAX scanner, by ImageMagick?

http://unix.stackexchange.com/questions/108613/how-do-you-binarize-a-colored-image — Ciro Santilli OurBigBook.com, Sep 25 '15 at 19:35
No ImageMagick requirement: http://superuser.com/questions/75373/convert-color-photos-of-documents-to-good-black-and-white-bitonal-images — Ciro Santilli OurBigBook.com, Sep 26 '15 at 08:21

score 22 · Accepted Answer · edited Aug 23 '14 at 10:19

22

However, if you want two colors only (black and white), then you need to threshold. For example, to select the color where above will be white and below will be black.

convert <input> -threshold xx% <output>

where xx is in range 0-100 (for percent).

edited Aug 23 '14 at 10:19

msnfreaky

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answered Mar 28 '12 at 10:35

Oliver Salzburg

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score 19 · Answer 2 · answered Mar 24 '15 at 18:08

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Dithering is clearer and more fax-like than a threshold cutoff:

convert <input> -monochrome <output>

For a less contrasty but more information-preserving kind of dithering, use:

convert <input> -remap pattern:gray50 <output>

(Docs)

answered Mar 24 '15 at 18:08

hemflit

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Note that if the output looks low-quality, you may need to set the density higher using the flag `-density 150` (the 150 is a dpi value) – David Fraser Dec 22 '15 at 15:22
Good answer as it gives 2 options that produce different output and you can choose the best result – golimar Jul 31 '21 at 10:38

score 9 · Answer 3 · edited Mar 17 '23 at 07:19

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According to this answer here:

If you have imagemagick installed:

true grayscale only:

convert source.jpg -colorspace Gray destination.jpg

true black and white:

convert source.jpg -monochrome destination.jpg

separate into gray channels:

convert source.jpg -separate destination.jpg

edited Mar 17 '23 at 07:19

Gabriel Staples

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answered Nov 21 '15 at 14:08

score 0 · Answer 4 · answered Jun 27 '16 at 13:18

I believe that Netpbm's pamthreshold is a much faster and more flexible solution.

For TIFF files, I do

 $ tifftopnm test.tiff | pamthreshold | pamtotiff > bitonal.tiff

For Jpeg files you can do

$ jpegtopnm test.jpeg | pamthreshold | pamtotiff > bitonal.tiff

Pamthreshold is rather powerful (take a look at its man page).

How to convert a photo to a black and white image by ImageMagick?

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