Page-by-page PDF diff tool

diffpdf.sh

#!/bin/bash

# usage: diffpdf.sh fidle_1.pdf file_2.pdf

# requirements:
# - ImageMagick
# - Poppler's pdftoppm and pdfinfo tools (works with 0.18.4 and 0.41.0,
#                                         fails with 0.42.0)
#   (could be replaced with Ghostscript if speed is
#    not important - see commented commands below)

DIFFDIR="pdfdiff"                        # directory to place diff images in
MAXPROCS=$(getconf _NPROCESSORS_ONLN)    # number of parallel processes

pdf_file1=$1
pdf_file2=$2

function diff_page {
    # based on http://stackoverflow.com/a/33673440/438249
    pdf_file1=$1
    pdf_file2=$2
    page_number=$3
    page_index=$(($page_number - 1))

    # 2+x faster
    (cat $pdf_file1 | pdftoppm -f $page_number -singlefile -gray - | convert - miff:- ; \
     cat $pdf_file2 | pdftoppm -f $page_number -singlefile -gray - | convert - miff:- ) | \
    convert - \( -clone 0-1 -compose darken -composite \) \
            -channel RGB -combine $DIFFDIR/$page_number.jpg

    # 2x faster (breaks when using TIFF format instead of JPEG, and PNG is slow)
#    (pdftocairo -f $page_number -singlefile -jpeg $pdf_file1 -gray - | convert - miff:- ; \
#     pdftocairo -f $page_number -singlefile -jpeg $pdf_file2 -gray - | convert - miff:- ) | \
#    convert - \( -clone 0-1 -compose darken -composite \) \
#            -channel RGB -combine $DIFFDIR/$page_number.jpg

    # 1x (using Ghostscript for PDF to bitmap conversion)
#    convert -respect-parenthesis \
#            \( $pdf_file1[$page_index] -flatten -colorspace gray \) \
#            \( $pdf_file2[$page_index] -flatten -colorspace gray \) \
#            \( -clone 0-1 -compose darken -composite \) \
#            -channel RGB -combine $DIFFDIR/$page_number.jpg

#    compare $pdf_file1[$page_index] $pdf_file2[$page_index] \
#            -highlight-color blue pdfdiff/$page_number.png

    if (($? > 0)); then
        echo "Problem running pdftoppm or convert!"
        exit 1
    fi
    grayscale=$(convert pdfdiff/$page_number.jpg -colorspace HSL -channel g -separate +channel -format "%[fx:mean]" info:)
    if [ "$grayscale" != "0" ]; then
        echo "page $page_number ($grayscale)"
        return 1
    fi
    return 0
}

function num_pages {
    pdf_file=$1

    pdfinfo $pdf_file | grep "Pages:" | awk '{print $2}'
}

function minimum {
    echo $(( $1 < $2 ? $1 : $2 ))
}

# guard agains accidental deletion of files in the root directory
if [ -z "$DIFFDIR" ]; then
    echo "DIFFDIR needs to be set!"
    exit 1
fi

pdf1_num_pages=$(num_pages $pdf_file1)
pdf2_num_pages=$(num_pages $pdf_file2)

min_pages=$(minimum $pdf1_num_pages $pdf2_num_pages)

if [ "$pdf1_num_pages" -ne "$pdf2_num_pages" ]; then
    echo "PDF files have different lengths ($pdf1_num_pages and $pdf2_num_pages)"
    rc=1
fi

if [ -d "$DIFFDIR" ]; then
    rm -f $DIFFDIR/*
else
    mkdir $DIFFDIR
fi


# get exit status from subshells http://stackoverflow.com/a/29535256/438249

function wait_for_processes {
    local rc=0

    while (( "$#" )); do
        # wait returns the exit status for the process
        if ! wait "$1"; then
            rc=1
        fi
        shift
    done
    return $rc
}

function howmany() {
    echo $#
}

rc=0
pids=""
for page_number in `seq 1 $min_pages`;
do
    diff_page $pdf_file1 $pdf_file2 $page_number &
    pids+=" $!"
    if [ $(howmany $pids) -eq "$MAXPROCS" ]; then
        if ! wait_for_processes $pids; then
            rc=1
        fi
        pids=""
    fi
done

if ! wait_for_processes $pids; then
    rc=1
fi

exit $rc

Thanks for the script. Works great. However I would make one change.

Currently the changes for first file given on the command line is colored red and the second is colored green. When creating patches using git or diff the original file comes first so that in a colored diff output the subtractions are colored red and the additions are colored green.
With the script the way it is now with the original file first, the colors for additions or subtractions are reversed.

One other idea would be to convert the jpgs to a pdf and remove the images and folder.
The following would create a PDF diff. The output filename could be specified on the command line after the two files to compare.
convert pdfdiff/*.jpg +compress output.pdf

@frederickjh You can find an improved Python version of this script here: diffpdf.py. The two changes you suggest aren't included, however. I'm using this script for regression testing, and creating a PDF requires significant extra time overhead, so that's why that is not included. I agree with your other suggestion, but I will probably not be implementing that any time soon, I'm afraid. Pull request are welcomed ;-)

Just an minor observation that in many industrial applications Red/Green Colour use was historically
RED = PROPOSED CHANGE / ADDITION / ADDED (AKA redlined)
GREEN = (TO)GO i.e.. REMOVAL/REMOVED
BLUE = (TOBE) MODIFIED / MOVED

@GitHubRulesOK Thanks for the input. I didn't know that! When I first wrote this script, I probably just reasoned: green good, red bad :-)