22

Similarly to this question:

Convert a PDF to greyscale on the command line in FLOSS?

I have a PDF-document and want to convert it to pure black and white. So I want to discard halftones. To convert to grayscale with ghostscript I can use this command:

gs \
 -sOutputFile=output.PDF \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -dProcessColorModel=/DeviceGray \
 -dCompatibilityLevel=1.4 \
  input.PDF < /dev/null

What do I have to change to get monochrome e.g. only the colors black and white and no halftones?

8

The last suggestion indeed only converts to grayscale and then only works if the underlying doc uses setrgbcolor. This did not work for me, since I had a doc, that used setcolor.

I had success with redefining setcolor to always set the color to 0,0,0:

gs -o <output-file.pdf> -sDEVICE=pdfwrite \
-c "/osetcolor {/setcolor} bind def /setcolor {pop [0 0 0] osetcolor} def" \
-f <input-file.ps>

It has been 15+ years since I did any PostScript hacking, so the above may be lame, incorrect or even accidental - if you know how to do better, please suggest.

| improve this answer | |
  • It should be {setcolor} rather than {/setcolor} since PostScript uses no slash when procedures are called during bind. Other than that: Great answer – thank you. – Hermann Aug 17 at 17:21
3

I am not sure if the following suggestion will work... but it may be worth to try out:

  1. convert the PDF to PostScript using the simple pdf2ps utility
  2. convert that PostScript back to PDF while using a re-defined /setrgbcolor PostScript operator

These are the commands:

First

  pdf2ps color.pdf color.ps

This gives you color.ps as output.

Second

gs \
-o bw-from-color.pdf \
-sDEVICE=pdfwrite \
-c "/setrgbcolor{0 mul 3 1 roll 0 mul 3 1 roll 0 mul 3 1 roll 0 mul add add setgray}def" \
-f color.ps
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  • I tried this and was still left with shades of gray. niklasfi wants monochrome. – frabjous Jan 23 '11 at 15:57
3

It's not ghostscript, but with imagemagick this is quite simple:

 convert -monochrome input.pdf output.pdf
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  • 11
    The resulting pdf quality is much much worse than original. – Hindol Jul 1 '13 at 7:52
  • convert -monochrome -denisty 600 ? – Ilia w495 Nikitin Jan 6 '18 at 6:42
  • This seems to do halftoning; I want all colors (but white) to be converted to black, regardless of darkness. – Gavin S. Yancey Oct 11 '18 at 6:00
2

for gray scale PDF:

By using GhostScript

IN PHP code, use this script

exec("'gs' '-sOutputFile=outputfilename.pdf' '-sDEVICE=pdfwrite' '-sColorConversionStrategy=Gray' '-dProcessColorModel=/DeviceGray' '-dCompatibilityLevel=1.4'  'inputfilename.pdf'",$output);

usefull url
http://www.linuxjournal.com/content/tech-tip-using-ghostscript-convert-and-combine-files

| improve this answer | |
1

This looks like it would work:

1) Convert the file to monochrome with gs

gs -sDEVICE=psmono \
  -dNOPAUSE -dBATCH -dSAFER \
  -sOutputFile=combined.ps \
  first.pdf \
  second.ps \
  third.eps [...]

3) Convert the Postscript file back to a PDF with ps2pdf or gs

(credit to: http://www.linuxjournal.com/content/tech-tip-using-ghostscript-convert-and-combine-files)

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1

I could not find out which procedure for color selection is used in the PDFs I am dealing with. This is why I convert to grayscale PostScript first:

gs -o gray.ps -sDEVICE=ps2write -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 -f colored.pdf

As the PDFs I struggle to print may contain confidential information which is cleverly "redacted" by having the color set to white, I need to employ some sort of thresholding. This is what I came up with:

gs -o thresholded.pdf -sDEVICE=pdfwrite -c "/osetgray {setgray} bind def /setgray {0.5 lt {0} {1} ifelse osetgray} def" -f gray.ps

For those (like me) unfamiliar with PostScript's stack programming style, this re-defines setgray as:

setgray(value) {
   original_setgray(value < 0.5 ? 0 : 1)
}
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0

For pure black and white PDF, you need to convert it into ps format then into PDF for postscript:

exec(" gs -sDEVICE=psmono  -dNOPAUSE -dBATCH -dSAFER  -sOutputFile=combined.ps  $pdf");

postscript to PDF -> black and white

exec(" gs -sDEVICE=pdfwrite   -dNOPAUSE -dBATCH -dSAFER  -sOutputFile=file_pdf.pdf  filename.ps");
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  • Ghostscript 9.50 complains "Unknown device: psmono" – Massimo Feb 6 at 17:15
0

I had to modify the solution suggested by Surge (above) a little bit for my file:

Step 1: Convert the coloured.pdf to coloured.ps

gswin64c -dNOPAUSE -dBATCH -sDEVICE=ps2write -sOutputFile=coloured.ps coloured.pdf

Step 2: Convert the coloured.ps to blackandwhite.pdf

gswin64c ^
-dBATCH -dNOPAUSE -q ^
-sOutputFile=blackandwhite.pdf ^
-sDEVICE=pdfwrite ^
-c "/osetrgbcolor {setrgbcolor} bind def /setrgbcolor {pop pop pop 0 0 0 osetrgbcolor} def" ^
-f coloured.ps

I did not have any success with setcolor operator as suggested by Surge. So I decided to play with other operators that can set colour is postscript like setgray, setrgbcolor, setcmykcolor, etc.

What I understand is that code in quotes following -c switch is postscript. It tells to bind the original definition of setrgbcolor with a new custom operator called osetrgbcolor . Now define a new instance setrgbcolor that pops the 3 inputs expected by original setrgbcolor and replace them with 0 0 0 i.e. red=0 green=0 blue=0. Thus 0 0 0 is passed to the operator osetrgbcolor custom defined earlier

PS1: The above code was implemented in windows command prompt

PS2: I was a total stranger to Postscript coding. I got a jumpstart from youtuber "John's Basement" in the video series Postscript Tutorial. I referred Adobe's Postscript Language Reference to understand the operator setrgbcolor and operands that it accepted.

| improve this answer | |

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