1 files changed, 58 insertions, 0 deletions
diff --git a/image_diff.pl b/image_diff.pl
new file mode 100644
index 0000000..9199f61
--- /dev/null
+++ b/image_diff.pl
@@ -0,0 +1,58 @@
+#!/usr/bin/perl -w
+
+# given 2 images of the same size, create a 3rd image, with the pixel
+# at each (x,y) transparent if it's the same RGB color in both images,
+# or set to the color in the 2nd image if they're different.
+
+# the 2nd image should be an edited version of the 1st image, and the
+# output image can be displayed overlaid on the 1st image to show what
+# the 2nd image looked like.
+
+use Image::Magick;
+
+sub die_usage {
+	die "usage: $0 <input-image1> <input-image2> <output-image>]\n";
+}
+
+sub readimage {
+	my $file = shift;
+	my $i = new Image::Magick;
+	my $r = $i->Read($file);
+	die "$r\n" if $r;
+	warn "read $file, OK\n";
+	return $i;
+}
+
+die_usage() if @ARGV != 3;
+$a = readimage(shift);
+$b = readimage(shift);
+
+$w = $a->Get('width');
+$h = $a->Get('height');
+
+if($w != $b->Get('width') || $h != $b->Get('height')) {
+	die "input images are not the same pixel size\n";
+}
+
+$out = Image::Magick->new(size => $w . 'x' . $h);
+$r = $out->ReadImage('xc:transparent');
+die "$r\n" if $r;
+
+# this is rather slow...
+for($y = 0; $y < $h; $y++) {
+	for($x = 0; $x < $w; $x++) {
+		my @ap = $a->GetPixel(map => "RGB", x => $x, y => $y);
+		my @bp = $b->GetPixel(map => "RGB", x => $x, y => $y);
+		if($ap[0] != $bp[0] || $ap[1] != $bp[1] || $ap[2] != $bp[2]) {
+			push @bp, 65535; # fully opaque
+		} else {
+			@bp = (0, 0, 0, 0); # transparent (and black)
+		}
+		$r = $out->SetPixel(map => "RGBA", x => $x, y => $y, color => \@bp);
+		die "$r\n" if $r;
+	}
+}
+
+$r = $out->Write($ARGV[0]);
+die "$r\n" if $r;
+warn "wrote to $ARGV[0]\n";