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I have several RGB images that I have to convert to CMYK for a paper submission. I have a Photoshop Elements 9 license but it doesn't support CMYK.

I remember having tried with ImageMagick on Linux but never managed to generate a result with the same colors. Last time I sent the files to a friend with a Photoshop license to convert.

Is there a free or cheap tool on Mac OS X, Linux or Windows that will allow me to do the job?

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2 Answers 2

up vote 6 down vote accepted

It seems you can use ImageMagick to convert from RGB to CMYK if you use the appropriate color profiles

For example if the source image contains a color profile

convert rgb_image.jpg -profile USCoat.icm cmyk_image.jpg

otherwise

convert rgb_image.jpg +profile icm \
                      -profile sRGB.icc  -profile USCoat.icm cmyk_image.jpg

But you must use appropriate color profiles, sRGB and USCoat are just examples.

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1  
The profiles are the trick. Converting between color systems isn't exact. This is something that the printer should be handling so they can get the best conversion results out of their equipment. –  afrazier Dec 31 '11 at 17:48
    
@afrazier Yes I know but I trying to explain the publisher that converting it without the right profile didn't bring anything. –  Matteo Dec 31 '11 at 17:52
    
@RedGrittyBrick Any suggestion on a more or less standard profile that will allow me to submit the pictures avoiding to make a mess? –  Matteo Dec 31 '11 at 17:53
    
@RedGrittyBrick Just to be sure: the image has an sRGB profile. If I do convert rgb.tiff -colorspace cmyk cmyk.tiff will it work (keeping the same profile)? Am I doing something wrong? –  Matteo Dec 31 '11 at 18:08
    
@Matteo: Use ImageMagick's identify -verbose rgb.tiff | egrep "Colorspace|Profile-" to check the color profile of the file (if any). As I understand it, the profile you specify also determines the colorspace of the output file, you can't produce a CMYK image using an RGB color profile. If your printer can give you a sample image, you can extract a color profile from it. The documentation I linked to gives examples. Whether this will produce the results you seek I do not know. –  RedGrittyBrick Jan 1 '12 at 11:51

Google:

RGB to CMYK conversion

and you'll find several online converters.

The larger problem, however, is that the publisher is clueless. Converting from RGB to CMYK should be done by whoever controls the printing process, not by random users using random software and color profiles. You can't control that, I realize, but at least be aware that you're dealing with someone who doesn't know what they're doing.

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Yes I know. I already tried to convince them without any luck. I was able to find many tools to convert values and a few to convert the images (e.g., rgb2cmyk.org or cmykconverter.com) but I will not manage to fit the files in the size limitation (>20MB) –  Matteo Dec 31 '11 at 17:58
    
Apparently the GIMP can do RGB to CMYK conversion with a plug-in, cue.yellowmagic.info/softwares/separate-plus/index.html –  Steve Rindsberg Jan 1 '12 at 21:20

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