Pngtricks is a small selection of scripts for working with png files. These
were originally written to understand and then reproduce PNG images that
produced fundamentally different output when viewed in different programs.

It turned out that this effect was a result of extreme values in aGAMA chunks,
which currently (2007-03-18) appear to be ignored by most common non-webbrowser
applications for image display purposes, but are honored by the common 
graphical webbrowsers.

schroedipnger.py is a script that duplicates this effect given two input
images; note that the result will have the size of an intersection of the
two images, and the actual quality will depend heavily on how exactly the 
original images looked. In general, large areas of similar colors seem to favor
the production of visual artifacts.