[cairo] A better image comparison tool

Antoine Azar cairo at antoineazar.com
Mon Sep 22 11:53:57 PDT 2008

Hey all,


this is something we discussed during the Cairo Summit, and I wanted to take
some feedback and ideas on the list before I start working on it.


The idea is to have a better image comparison tool in the Cairo test suite
instead of simply checking for any pixel discrepancy. Carl pointed out we
currently have several reference image sets to compensate for differences
across backends. Such a tool would ideally eliminate these multiple images
as much as possible. Another advantage of such a tool would be some
integration in the build/test bot machines Mozilla is setting up.  If a
given patch changes the results of a render in a (new) correct way, instead
of seeing a broken test suite, we would see e.g. a 90% similarity score for
the given test.


I am considering a couple approaches. The first one I want to try out is a
pretty simple square sum of differences or sum of absolute differences.
There are more fancy algorithms like cross correlation and mutual
information,  which I'll probably try out too. We can then set custom
thresholds for what should appear as "OK", "Warning" and "Error". 




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