For the use of "null:", see http://www.imagemagick.org/script/comma ... php#layers , method "composite".
When you have a difference image that is black where there is no difference, you can process that image, eg "-auto-level", or "-fill white +opaque black" to turn all non-black into white.
Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".