#!/usr/bin/perl -w
#
# Script to scan a directory (presumed to be an exploded ebook of some kind)
# and replace all grayscale .jpeg and .jpg images with .png images that use
# transparency. The idea is that they will look identical on a white
# background, but actually take on the background color or image if you set
# something other than white in your ebook.
#
# Any text files in the directory are scanned for the names of the image
# files and the references are changed to point to the new .png files.
#
# No arguments, just run it while sitting in the exploded ebook directory.
#
use strict;

use File::Find;
use File::Basename;

my @jpgs;

my @txts;

my @opfs;

my $tmpdir="png$$";

sub check_gray {
   my $jpg = shift;
   my $fh;
   open($fh, '-|',
        'convert', $jpg,
        '(', '+clone', '-colorspace', 'gray', ')',
        '-compose', 'difference', '-composite', '-colorspace', 'gray',
        '-format', '%[maximum]', 'info:');
   my $factor = <$fh>;
   close($fh);
   chomp($factor);
   return ($factor=~/^\d+$/) && (($factor + 0) < 5000)
}

sub scanfile {
   if (-f $_) {
      # want plain files only, no directories also get rid of the leading ./
      my $dotfree = substr($File::Find::name,2);
      if ((/\.jpeg$/) || (/\.jpg$/)) {
         # This should be one of the .jpg files we are looking for.
         if (check_gray($_)) {
            push(@jpgs, $dotfree);
         } else {
            print "$dotfree is not grayscale\n";
         }
      } elsif (-T $_) {
         # This is some other file in the book which might contain
         # the name of one of the files we are going to change.
         push(@txts, $dotfree);
         if ($_=~/\.opf$/) {
            push(@opfs, $_);
         }
      }
   }
}

find(\&scanfile, ".");

if (scalar(@jpgs) == 0) {
   print "No jpg images found.\n";
   exit(0);
}

sub fix_opfs {
   # Any .opf files may well have .png file references which claim
   # to have mime type image/jpeg. This final fix replaces those
   # image/jpeg mime types with image/png.
   my $opf = shift;
   my $temp = "tempopf.$$";
   my $inf;
   my $outf;
   my $update=0;
   local $_;
   if (open($inf, '<', $opf)) {
      if (open($outf, '>', $temp)) {
         while (<$inf>) {
            if (/.*item.*\.png.*image\/jpeg/) {
               s/image\/jpeg/image\/png/;
               $update=1;
            }
            print $outf $_;
         }
         close($outf);
      }
      close($inf);
   }
   if ($update) {
      unlink($opf);
      rename($temp,$opf);
   } else {
      unlink($temp);
   }
}

# Link all the grayscale jpg files into a temp directory

mkdir($tmpdir);
my $f;
foreach $f (@jpgs) {
   my $b = basename($f);

   # The gimp color to alpha filter only works on RGB images so make sure
   # any grayscale images get converted to RGB by using the convert command
   # to copy the RGB versions to the temp directory.

   system("convert", "$f", "-depth", "8", "-type", "TrueColor", "$tmpdir/$b");
}

# Convert all the files in the temp directory

chdir($tmpdir);
system("gimp", '-i', '-b', '(batch-set-alpha "*" )', '-b', '(gimp-quit 0)');
chdir("..");

# Now rename all the temp files

my @replacepats;

foreach $f (@jpgs) {
   my($b,$d,$s) = fileparse($f, qr/\.[^.]*/);
   if (-f "$tmpdir/${b}$s.png") {
      link("$tmpdir/${b}$s.png", "${d}$b.png");
      push(@replacepats, $f, "${d}$b.png");
      unlink($f);
      print "$f replaced with ${d}$b.png\n";
   } else {
      print "Hey! $f was not converted to a .png file!\n";
   }
}
system("rm -rf $tmpdir");

push(@replacepats, '--', @txts);

system("replace", @replacepats);

if (scalar(@opfs) > 0) {
   foreach $f (@opfs) {
      fix_opfs($f);
   }
}