I had compare working fine, but after an upgrade to Ubuntu 17.04 it started showing "transparent" as black.
Here is a command example:
Code: Select all
compare -highlight-color magenta -lowlight-color transparent a.png b.png -compose src -colorspace sRGB diff.png
Code: Select all
compare --version
Version: ImageMagick 6.9.7-4 Q16 x86_64 20170114 http://www.imagemagick.org
Copyright: © 1999-2017 ImageMagick Studio LLC
License: http://www.imagemagick.org/script/license.php
Features: Cipher DPC Modules OpenMP
Delegates (built-in): bzlib djvu fftw fontconfig freetype jbig jng jpeg lcms lqr ltdl lzma openexr pangocairo png tiff wmf x xml zlib
For comparison, attaching the output also from a different machine
a.png:

b.png:

diff.png:

diff on a different computer:
