How to convert a photo to a black and white image by ImageMagick?
According to this forum post:
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).
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)
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