Comparison of two pdf files

Question

I need to compare the contents of two almost similar files and highlight the dissimilar portions in the corresponding pdf file. Am using pdfbox. Please help me atleast with

Answer 1

You can do the same thing with a shell script on Linux. The script wraps 3 components:

1. ImageMagick's compare command
2. the pdftk utility
3. Ghostscript

It's rather easy to translate this into a .bat Batch file for DOS/Windows...

Here are the building blocks:

pdftk

Use this command to split multipage PDF files into multiple singlepage PDFs:

pdftk  first.pdf  burst  output  somewhere/firstpdf_page_%03d.pdf
pdftk  2nd.pdf    burst  output  somewhere/2ndpdf_page_%03d.pdf

compare

Use this command to create a "diff" PDF page for each of the pages:

compare \
       -verbose \
       -debug coder -log "%u %m:%l %e" \
        somewhere/firstpdf_page_001.pdf \
        somewhere/2ndpdf_page_001.pdf \
       -compose src \
        somewhereelse/diff_page_001.pdf

Note, that compare is part of ImageMagick. But for PDF processing it needs Ghostscript as a 'delegate', because it cannot do so natively itself.

Once more, pdftk

Now you can again concatenate your "diff" PDF pages with pdftk:

pdftk \
      somewhereelse/diff_page_*.pdf \
      cat \
      output somewhereelse/diff_allpages.pdf

Ghostscript

Ghostscript automatically inserts meta data (such as the current date+time) into its PDF output. Therefore this is not working well for MD5hash-based file comparisons.

If you want to automatically discover all cases which consist of purely white pages (that means: there are no visible differences in your input pages), you could also convert to a meta-data free bitmap format using the bmp256 output device. You can do that for the original PDFs (first.pdf and 2nd.pdf), or for the diff-PDF pages:

 gs \
   -o diff_page_001.bmp \
   -r72 \
   -g595x842 \
   -sDEVICE=bmp256 \
    diff_page_001.pdf

 md5sum diff_page_001.bmp

Just create an all-white BMP page with its MD5sum (for reference) like this:

 gs \
   -o reference-white-page.bmp \
   -r72 \
   -g595x842 \
   -sDEVICE=bmp256 \
   -c "showpage quit"

 md5sum reference-white-page.bmp