Converting a PDF to black & white with ghostscript

Question

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?

Answer 1

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.

Answer 2

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

Answer 3

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

 convert -monochrome input.pdf output.pdf