It's quite easy to find out about the color weighting of IM: You just
have to create a RGB test image:
convert -size 200x200 xc:red r.gif
convert -size 200x200 xc:green1 g.gif
convert -size 200x200 xc:blue b.gif
montage r.gif g.gif b.gif rgb.gif
and then convert it into grayscale:
convert -colorspace Gray rgb.gif gray.jpg
(You need to switch to JPEG at this point as it supports grayscale
colors, while GIF doesn't.)
Loading the Result into GIMP you will see that the weighting is
R: 30%
G: 59%
B: 11%
which is quite close to 30-60-10. This does not seem to give reason for
drastic differences.
I think that the proposed command
convert -modulate '100,0,100' color-image.jpg de-saturated.jpg
will not lead to good results as you may put on a test with that RGB
image: It will turn into pure white.
For test purposes you may establish any other color weighting by use of
the -fx operator, for instance:
convert -fx '0.3*r+0.4*g+0.3*b' rgb.gif gray.jpg
Wolfgang Hugemann
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