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Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black?

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In article <6d0f8a6c-3430-4b2f-8852-
ddb0e772e4ac@v8g2000yqb.googlegroups.com>, sd@imi.aau.dk says...

> > Its also possible to set up an under colour removal function which
> > significantly affects how RGB is converted to CMYK (this is covered in
> > the PostScript Language Reference Manual).
> >
> 
> Thanks for noting this - I was somewhat aware that postscript language
> can also "process", but I am completely ignorant about its scope. Re:
> the DeviceGray CMYK workflow, I was intuitively trying to follow that
> in the above "monster" cmdline too (as in: force all colors to
> grayscale during conversion [in conversion colorspace], and write out
> CMYK values based on these grayscale ones, which hopefully end up only
> on the K plate) -- but I couldn't get `tiffsep` to confirm that.
> 
> 
> > If you can post a (small!) example file, preferably a single page, to
> > some publicly accessible URL I could take a look.
> >
> 
> Sure, here are the pdf's of the slides mentioned in the OP:
> 
> http://sdaaubckp.sf.net/post/img/blah-slide.pdf
> http://sdaaubckp.sf.net/post/img/blah-slideP.pdf

I wasn't able to do this conversion in a single pass using the 
Ghostscript PDF interpreter, because it uses the setrgbcolor directly 
from systemdict, so it doesn't allow for replacement. 

Instead I first converted your files to PostScript using the ps2write 
device:

gs -sDEVICE=ps2write -sOutputFile=./out.ps ./blah-slide.pdf

Then I created a simple replaement routine, and stored it in a file 
called HackRGB.ps:

%!
/oldsetrgbcolor /setrgbcolor load def
/setrgbcolor {
(in replacement setrgbcolor\n) print
				%% R G B
  1 index 1 index       %% R G B G B 
  eq {                  %%
     2 index 1 index    %% R G B R B
     eq {
                        %% Here if R = G = B
      pop pop           %% remove two values
      setgray
     } {
       oldsetrgbcolor   %% set the RGB values
     } ifelse
  }{ 
    oldsetrgbcolor      %% Set the RGB values
  }ifelse
} bind def

This replaces the setrgbcolor operator with a routine which tests the 
RGB value and if all components are equal it replaces it with a call to 
setgray using just one of the components. (BTW you can remove the line 
ending in 'print', its just there so that you can see something is 
happening ;-)

I then converted the PostScript file back to PDF, but using this code:

gs -sDEVICE=pdfwrite -sOutputFile=./out.pdf ./HackRGB.ps ./out.ps

This results in a PDF file where the text is in a shade of gray. This 
*ought* to be acceptable to your print shop, because gray should map 
straight to the K channel of CMYK.

If for some reason that isn't acceptable, you could replace the 
'setgray' with '0 0 0 4 -1 roll setcmykcolor' which uses CMYK directly.

NOTE! This only affects linework (text, vectors), only affects linework 
using RGB and will only ocnvert that to gray if the R, G and B values 
are identical. Images, shadings and potentially other object types will 
not be affected.

I should also mention that going from PDF to PostScript and back to PDF 
is a potentially lossy process which can introduce errors and odd 
artefacts, you should check files carefully after this conversion. I 
haven't tested this code particularly.

If you print directly to PostScript then you can eliminate one 
conversion step, which is probably worthwhile.


			Ken

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PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-05 22:49 -0700
  Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? Matti Vuori <xmvuori@kolumbus.fi> - 2011-06-06 08:41 +0000
  Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? Helge Blischke <h.blischke@acm.org> - 2011-06-06 11:26 +0200
    Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-06 04:29 -0700
  Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ken <ken@spamcop.net> - 2011-06-06 14:05 +0100
    Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-06 08:34 -0700
      Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ken <ken@spamcop.net> - 2011-06-06 17:28 +0100
      Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ken <ken@spamcop.net> - 2011-06-07 10:21 +0100
        Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-09 05:28 -0700
          Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ken <ken@spamcop.net> - 2011-06-09 14:00 +0100
            Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-09 06:48 -0700
              Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ken <ken@spamcop.net> - 2011-06-09 16:06 +0100
          Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? tlvp <mPiOsUcB.EtLlLvEp@att.net> - 2011-09-16 18:33 -0400
    Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? sdaau <sd@imi.aau.dk> - 2011-06-06 09:17 -0700
      Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? Helge Blischke <h.blischke@acm.org> - 2011-06-06 19:30 +0200
        Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ole.hoppe@gmail.com - 2012-01-25 06:40 -0800
          Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? Helge Blischke <h.blischke@acm.org> - 2012-01-25 16:09 +0100
            Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ole.hoppe@gmail.com - 2012-01-26 06:41 -0800
              Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? Helge Blischke <h.blischke@acm.org> - 2012-01-26 21:07 +0100
              Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ole.hoppe@gmail.com - 2012-01-26 12:57 -0800
            Re: PDF to PDF (gs?): rich RGB black to plain K (CMYK) black? ole.hoppe@gmail.com - 2012-01-26 12:58 -0800

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