Converting (any) PDF to black (K)-only CMYK

Question

This is related to:

• Converting PDF to CMYK (with identify recognizing CMYK).
• Script (or some other means) to convert RGB to CMYK in PDF?

... but a bit more specific here: say I have an RGB PDF, where the text color is "rich black" (R:0 G:0 B:0 gone to C:100 M:100 Y:100 K:100), and diverse images and vector graphics.

I would like to convert this to a CMYK PDF, using a free command line tool (so it is batch scriptable under Linux), which

• has contents only in the black (K) channel:
  • Preserves vector graphics (+ text glyphs) - colors become grayscale in black (K) channel only
  • Images get converted to grayscale in black (K) channel only

Thanks in advance for any answers,
Cheers!

Answer 1

As hinted in my comment to @Mark Storer, it turns out that forcing a gray print only on the K plate in CMYK, may not be so trivial ... I guess it depends much on what is being used as "preflight" preview device - for Linux, the only thing I can find is ghostscript with tiffsep, which is what I use for 'sanity check' regarding CMYK separations.

Anyways, I got a lot of help in this thread on comp.lang.postscript:

• PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? - comp.lang.postscript | Google Groups

... and one workflow that works for me is:

• Convert PDF to PS using ghostscript's ps2write
• Use ghostscript to convert this PS back to PDF, while executing replacement functions in HackRGB-cmyk-inv.ps
• Use ghostscript's tiffsep to check actual separations

 

In respect to, say, this PDF generated by OpenOffice: blah-slide.pdf, the command lines would be:

# PDF to PS using `ps2write` device of `ghostscript`
gs \
   -dNOPAUSE \
   -dBATCH \
   -sDEVICE=ps2write \
   -sOutputFile=./blah-slide-gsps2w.ps \
    ./blah-slide.pdf 

# PS to PDF using replacement function in HackRGB-cmyk-inv.ps
gs \
   -dNOPAUSE \
   -dBATCH \
   -sDEVICE=pdfwrite \
   -sOutputFile=./blah-slide-hackRGB-cmyk-inv.pdf \
    ./HackRGB-cmyk-inv.ps \
    ./blah-slide-gsps2w.ps

# check separations
gs \
   -dNOPAUSE \
   -dBATCH \
   -dSAFER \
   -sDEVICE=tiffsep \
   -dFirstPage=1 \
   -dLastPage=1 \
   -sOutputFile=p%02d.tif \
    blah-slide-hackRGB-cmyk-inv.pdf \
\
&& eog p01.tif 2>/dev/null 

This should only work on RGB values where R=G=B (and hopefully grayscale values), and only on text colors, and it also flattens text information - but it should be possible to confirm via tiffsep that the text indeed ends up only on the K plate.

As mentioned in the newsgroup post, this is not extensively tested, but looks promising so far...
Cheers!

Answer 2

As an improvement to sdaau's great answer, I can recommend using pdftops from xpdf for converting pdf to ps, instead of ghostscript ps2write, because the latter e.g. causes the font to become staircasey, and is said to not preserve the original pdf accurately. Compare by zooming into text areas of the resulting pdfs.

Answer 3

I suggest you convert the PDF using GS twice. Once to a Shades Of Gray colorspace, and then to CMYK.

I'm not sure it'll work, but I'd be a bit surprised if it didn't. G->CMYK sounds like a brain-dead X -> 0 0 0 X conversion. At least if you stick to "device gray" and "device CMYK" instead of some calibrated color space that'll tweak things this way and that.