Converts a PDF obtained from scanning into a fax-like monochrome PDF. Assumes that the PDF consists of only a sequence of page-filling images. Requires my other gist imagemagick-scan-to-mono.sh

imagemagick-scan-pdf-to-mono.sh

#!/usr/bin/env bash
# -*- mode: sh; coding: us-ascii-unix -*-

source libstacktrace || true
# set -e -u -E

MANUAL="
Usage: $0 [options] INPUT OUTPUT
       $0 --inplace [options] INPUT

Converts a scan-pdf (assuming one image per page) to monochrome.

When OUTPUT is not given, replace the input file instead and create a
backup file.

-f INT, --from-page INT
    Process only pages with page number >= INT

-t INT, --to-page INT
    Process only pages with page number <= INT

-P, --parallel INT
    Process INT pages in parallel each. 

-v, --verbose / +v, --noverbose
    Enables/Disables verbose reporting.

-i, --inplace 
    Instead of producing OUTPUT, change INPUT inplace and create a
    backup file INPUT.bak. 

-d,--density FLOAT
    Instead of using 'pdfimages' for image conversion, use imagemagick
    with the given number passed to the '-density' flag. 
    
    Meant for pdfs that do NOT consist of one image per page and 
    nothing else. For files that do, this setting will cause a loss of
    quality!

-h, -?, --help
    Prints this message

"

vecho(){ $VERBOSE && echo "$@" || true; }
vmv(){ $VERBOSE && mv -v "$@" || mv "$@"; }

######### COMMAND LINE PARSING #######################################

declare VERBOSE=false
declare ARGS=()
declare PAGE_LIMIT_LOW=""
declare PAGE_LIMIT_HIGH=""
declare PARALLEL=1
declare INPLACE=false
declare PDFIMAGES=true

## Print manual
if [[ $# -eq 0 ]]; then
    echo "$MANUAL"
    exit 1
fi

## Getopt-style consumption of arguments ##
##
## Don't forget "shift", don't delete "--" and "*" cases.
while [[ $# -gt 0 ]]; do
    case "$1" in
	-h|-\?|--help)
	    echo "$MANUAL"
	    exit 0
	    shift ;;
	-v|--verbose)
	    VERBOSE=true
	    shift ;;
	+v|--no-verbose)
	    VERBOSE=false
	    shift ;;
	-f|--from-page)
	    PAGE_LIMIT_LOW="-f $2"
	    shift 2 ;;
	-t|--to-page)
	    PAGE_LIMIT_HIGH="-l $2"
	    shift 2 ;;
	-P|--parallel)
	    PARALLEL=$2
	    shift 2 ;;
	-i|--inplace)
	    INPLACE=true 
	    shift ;; 
        -d|--density)
            DENSITY=$2
            PDFIMAGES=false
            shift 2 ;;
	--)
	    shift
	    break ;;
	*) 
	    ARGS[${#ARGS[@]}]="$1"
	    shift ;; 
    esac
done

## Consume stuff remaining after -- ##
while [[ $# -gt 0 ]]; do 
    ARGS[${#ARGS[@]}]="$1"
    shift
done

## Note that ${ARGS[@]} is considered unbound if it is empty!

INFILE=$(readlink -m "${ARGS[0]}") 
if ! $INPLACE; then
    OUTFILE=$(readlink -m "${ARGS[1]}")
else
    OUTFILE=$(mktemp -t "$(basename INFILE .pdf).XXXXXXXXX.pdf")
fi
TMPDIR=$(mktemp -d)

vecho "Using work directory '$TMPDIR'."
cd "$TMPDIR"
vecho "Extracting images from '$INFILE'..."

## Cannot be parallelized, file-locking issue. 

if $PDFIMAGES; then 
    cmd="pdfimages -j $PAGE_LIMIT_LOW $PAGE_LIMIT_HIGH $(printf %q "$INFILE") page"
else
    cmd="convert -density $(printf %q "$DENSITY") $(printf %q "$INFILE") page-%03d.png" 
fi
# vecho "$cmd"
eval "$cmd" || true

find -name "page-*" -and -not -name "page-*-mono*" \
    | xargs -P $PARALLEL -I FILE sh -c "
      imagemagick-scan-to-mono.sh FILE FILE-mono.png \
         && convert FILE-mono.png -flatten FILE-mono.pdf \
         && { if $VERBOSE; then echo Finished file 'FILE'; fi; }
      rm FILE FILE-mono.png 
    "

vecho "Assembling PDF file '$OUTFILE'..."
pdftk page-*-mono.pdf cat output out.pdf 
mv out.pdf "$OUTFILE"
rm page-*-mono.pdf
rmdir "$TMPDIR" || ls -l

if $INPLACE; then
    i=0
    while [[ -e $(printf "%s.bak%03d.pdf" "$INFILE" "$i") ]]; do
	((i++))
    done
    vmv "$INFILE" "$(printf "%s.bak%03d.pdf" "$INFILE" "$i")"
    vmv "$OUTFILE" "$INFILE" 
fi