How can I convert an image to grayscale via the command line?

Question

How can I use sips, imagemagic, or another tool to convert an image to black and white (grayscale) via the command line?

Solution 1

If you have imagemagick installed,

convert source.jpg -colorspace Gray destination.jpg (true grayscale only)
convert source.jpg -monochrome destination.jpg (true black and white)
convert source.jpg -separate destination.jpg (separate into gray channels)

If you don't care about losing the original file: mogrify -colorspace Gray file.

Solution 2

use one of: -monochrome or -colorspace gray options for imagemagick (convert).

Comments

[Chiron]

Open built-in python: from PIL import Image ; Image.open('a.png').convert('L').save('grey.png') for those who prefer staying away from software installing.

[C--]

None of these actually convert a AxBx3 RGB image into a AxBx1 single channel grayscale image though.

[Tiago Franco]

The separate worked great for me.