#773552 graphicsmagick-imagemagick-compat: convert to cmyk leaves k channel empty

Package:
graphicsmagick-imagemagick-compat
Source:
graphicsmagick
Submitter:
Michael Below
Date:
2014-12-20 16:30:08 UTC
Severity:
normal
#773552#5
Date:
2014-12-19 20:12:31 UTC
From:
To:
Dear Maintainer,

I tried to use convert from graphicsmagick to convert an AdobeRGB image to CMYK, using the ECI profile "ISOcoated_v2_300_eci.icc" from the ECI website. It converted the image to use only the CMY colors, not CMYK. The K channel was there, but it was empty. Now I have converted the same image with the same ICC profile using the original imagemagick convert (6.8.9-9 Q16), and the K channel is doing it's part, with a max value of 0.959 and a mean of 0.797 (so it's not like the image has no black).

It looks like the conversion that is labelled CMYK converts only to CMY. That would be misleading (there is a seperate option for CMY). If CMYK can't be supported right now, please consider adding a warning, so that people don't produce botched CMYK conversions. Imagemagick is actually the recommended linux tool for that, so it is likely people will try to use graphicsmagick for it.

Thanks for your effort!

Michael
#773552#10
Date:
2014-12-20 16:15:10 UTC
From:
To:
After private discussion with Michael (and an exchange of image files)
it has been determined that the main problem he encountered is that
GraphicsMagick identify was not properly computing statistics for the
Black channel of CMYK images so only 0 was reported.  This is fixed by
Mercurial changeset 2739f0e0813d.

Other factors encountered were that the default rendering intent is
different between ImageMagick and GraphicsMagick, ImageMagick is a Q16
build while GraphicMagick is a Q8 build, and ImageMagick tries to
preserve the original JPEG quality settings by default, whereas
GraphicsMagick does not.

Bob
#773552#15
Date:
2014-12-20 16:15:10 UTC
From:
To:
After private discussion with Michael (and an exchange of image files)
it has been determined that the main problem he encountered is that
GraphicsMagick identify was not properly computing statistics for the
Black channel of CMYK images so only 0 was reported.  This is fixed by
Mercurial changeset 2739f0e0813d.

Other factors encountered were that the default rendering intent is
different between ImageMagick and GraphicsMagick, ImageMagick is a Q16
build while GraphicMagick is a Q8 build, and ImageMagick tries to
preserve the original JPEG quality settings by default, whereas
GraphicsMagick does not.

Bob