Making a PDF grayscale with ghostscript
A request from a friend made me face the problem of converting a color [[Portable Document Format|PDF]] into a [[grayscale]] one. Searching the web provided some ways of doing so with [[Adobe Acrobat]], via some obscure menu item somewhere.
However, the very same operation could be undertaken with free tools, such as [[ghostscript]]. I found a way to do it in the YANUB blog, and I will copy-paste it here, with a small modification.
Assuming we have a file called color.pdf, and we want to convert it into grayscale.pdf, we could run the following command (all in a single line, and omitting the “\” line continuation marks):
-sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH color.pdf
I prefer the above to YANUB’s version below (in red what he lacks, in blue what I lack), because a shell operation is substituted by some option(s) of the command we are running:
-sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray \
-dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH color.pdf < /dev/null
A sample [[Perl]] script to alleviate the tedious writing above:
use strict;
my $infile = $ARGV[0];
my $outfile = $infile;
$outfile =~ s/\.pdf$//;
$outfile = $outfile.”_gray.pdf”;
system “gs -sOutputFile=$outfile -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 -dNOPAUSE -dBATCH $infile”
Assuming we call the Perl script “togray.pl”, and that we have a color file “input.pdf”, we could just issue the command:
and we would get a grayscale version of it, named “input_gray.pdf”.
