Watermarking a PDF with ghostscript

Posted December 7th 2016

Transforming an existing PDF so that it gets something like 'Draft' stamped across each page is surprisingly easy with ghostscript. Shell script:

#!
# Add a watermark to a PDF file
#
# first argument: the filename
# second argument: the watermark text (default "watermark")

filename=$1
watermark=$2

if [ x$2 == x ]; then watermark="watermark"; fi

cat > /tmp/watermark.ps << HERE
<<
   /EndPage
   {
     2 eq { pop false }
     {
         gsave
         0.5 .setopacityalpha
         /Helvetica_Bold 120 selectfont
         .65 setgray 130 70 moveto 50 rotate ($watermark) show
         grestore
         true
     } ifelse
   } bind
>> setpagedevice
HERE

gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=watermarked_$filename /tmp/watermark.ps $filename

There are three tricks above. First, you have to know about the 'EndPage' hook in postscript. It's code executed at the end of each page. Second, you have to know that the parameter can be 0, 1 or 2, depending on the reason for the 'EndPage'. The postscript reference manual explains what the codes mean, but we're interested in all but 2. Third, there's the '.setopacityalpha' operator, which I think only works in PostScript 1.4, but that's OK because that's pretty standard now.

(The temporary file is ugly. Sorry.)