Re: comparing pdfs

[Sanjoy Mahajan <sanjoy@MIT.EDU>]

> Is there a way more quick and clean than  for cycle and ppm files?

The following shell script is not quick or clean, but it is thorough and
I use it to find changes from one version of a pdf file to the next.
For example, for my textbook page proofs, after I fix a bad line break,
I compare the new and most recent previous versions to check that the
fix has not created subsequent bad page breaks.

The script runs on GNU/Linux and requires pdftoppm (from xpdf or
poppler) and imagemagick (for the 'compare' utility).

It generates the comparison bitmaps in a /tmp directory.  The output
looks like:

/tmp/tmp.IWHtn0z7hk/diff-1.ppm   4250.32 (0.0648558)
/tmp/tmp.IWHtn0z7hk/diff-2.ppm   3429.2 (0.0523262)
/tmp/tmp.IWHtn0z7hk/diff-3.ppm   2890.33 (0.0441036)
/tmp/tmp.IWHtn0z7hk/diff-4.ppm   1455.9 (0.0222157)

where column 1 is the filename, which tells you which page is being
compared, column 2 is a measure of the difference between the two files
on that page, and column 2 is a normalized measure of column 2.  

To view the pages in order of most to least changed:

compare-pdfs a.pdf b.pdf | sort -nr -k2 | awk '{print $1}' | xargs feh -FV

I put this command in the Makefile for a project.

-Sanjoy

#! /bin/bash

# Usage: $0 file1.pdf file2.pdf
#   compares file1.pdf and file2.pdf by converting each page to bitmaps using
#   pdftoppm and then using the 'compare' ImageMagick utility
#
# Copyright 2007-2009 Sanjoy Mahajan.  Licensed under the GNU GPL version 2
# or (at your option) any later version.
#
# HISTORY
#   2009-09-30: Fix capture of dB output; don't use a viewer; use pdftoppm
#   2007-01-15: First version
#

dpi=144

if [ -z "$1" -o -z "$2" ]; then
  echo "Usage: $0 file1.pdf file2.pdf"
  exit 3
fi

# generate the many page images in a temporary directory
d=`mktemp -d`
pdftoppm -r $dpi $1 $d/one &
pdftoppm -r $dpi $2 $d/two &
wait

# find the union of the page numbers (in case one pdf has more pages)
pages=`ls $d/{one,two}-*.ppm | sed "s/.*-\([0-9][0-9]*\).ppm/\1/" | sort -un`

# compare each page
for p in $pages ; do
  if ! [ -e "$d/one-$p.ppm" ] ; then
    echo "$p: missing from $1"
    continue
  fi
  if ! [ -e "$d/two-$p.ppm" ] ; then
    echo "$p: missing from $2"
    continue
  fi
  echo -n "$d/diff-$p.ppm   "
  compare -metric mae $d/{one,two}-$p.ppm $d/diff-$p.ppm 2>&1
done
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