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?


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.

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  • 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

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:


  pdf2ps color.pdf color.ps

This gives you color.ps as output.


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

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

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

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This looks like it would work:

1) Convert the file to monochrome with gs

gs -sDEVICE=psmono \
  -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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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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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

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 ^
-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.

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