ImageMagick convert produces a darker CMYK PDF than PhotoShop

0
votes

The ImageMagick (IM) result of this command 

convert myRGB.png -colorspace cmyk cmyk.pdf <br>

is not as bright or as close to the screen colors as a Photoshop produced CMYK PDF. myRGB.png is a PNG file produced using GIMP.

I don't own Photoshop, and would like to stick with open source tools. The current Ubuntu release of of IM is 6.7.7. That IM version produces very dark, totally unusable, CMYK PDF.

I built 7.0.2-6 Q16 from source on Ubuntu 14.0.4, after also building LCMS package from source, and the above command works better, but the CMYK PDF as stated above is less bright and less close to the screen colors than the similar Photoshop output. E.g. blacks are not totally black; the sky color is dull blue instead of bright blue/cyan.

I've tried using ICC files downloaded from Adobe as in the following 

convert myRGB.png -colorspace cmyk -profile WebCoatedSWOP2006Grade5.icc cmyk.pdf

I've tried this command with all 14 Adobe ICC files and there is no difference in any of them. Although, I admit I do not understand under what circumstances the ICC comes into play or if it is appropriate to this problem at all.

The simple question is why does IM convert tool not match the Photoshop results for CMYK?
The second question is, if IM can't be made to do it: is there any open source tool or tools that can match the Photoshop results for producing a CMYK PDF from and RGB PNG?

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pdfimagemagickghostscriptcmyk

1 Answers

1
votes

There are two applications involved, as you presumably know since you tagged this with Ghostscript. You haven't said which version of Ghostscript you have installed but the first thing I would do is remove ImageMagick from the equation.

Find out whether IM is having Ghostscript produce RGB or CMYK output, my bet is that it is getting RGB from GS. You'll need to find out what Ghostscript command line IM is using and I can't tell you how to do that. Assuming that the Ghostscript output is RGB then this would explain why altering the IM settings makes no difference.

Proceeding on the assumption that the above is correct, use the png16m device in Ghostscript to produce RGB PNG files directly, this reduces the scope of the problem:

gs -sDEVICE=png16m -o out.png input.pdf

Now, you don't say what version of Ghostscript you have installed, but assuming its relatively recent you can look in the /ghostpdl/doc directory and find considerable information on using colour management in Ghostscript, the document GS9_Color_Management.pdf may be helpful. It will certainly give you a myriad of opportunities to alter the output.