如何将 RGB PDF 转换为带有纯黑色的 CMYK？

Question

I used instructions from here to convert an RGB PDF to CMYK using Ghostscript, and it's mostly OK except all the blacks are "rich" - they use not just K but also CMY inks.

Is there a way to convert such that all blacks are "flat" and just use K?

This is the code I used:

gs \
   -o test-cmyk.pdf \
   -sDEVICE=pdfwrite \
   -sProcessColorModel=DeviceCMYK \
   -sColorConversionStrategy=CMYK \
   -sColorConversionStrategyForImages=CMYK \
    test.pdf 

Answer 1

Assuming your PDF file actually does use RGB (and not an ICC profile embedded in the PDF) then, in order to get R=G=B->C==M=Y=0, K=R, you need to set up a custom ICC profile link.

You'll need to tell Ghostscript to use a custom RGB profile in place of default_rgb.icc and a cuustom CMYK profile in place of default_cmyk.icc. You'll need to ensure that the mapping RGB->XYZ->CMYK results in pure K when R=G=B.

There's documentation in the Ghostscript 'doc' folder on the various colour management settings, but most of these will only be effective when rendering, not when outputting a PDF file. About the only things you can usefully alter when outputting PDF is the input and output profiles.