Thanks for your input! :)

I have something like that in mind but I don't think that a complicated 
algorithm is needed here; we're basically working with two alpha bitmaps of 
(roughly) the same size. I would simply compare the bitmaps on a pixel-by-pixel 
basis, weigh the results by the max. possible difference and get the average of 
all differences. That way it would also be simple to add a threshold - may it 
be absolute or relative. In the end, the result can also be applied to a 
logarithmic scale - something that I would play around with to find the optimal 
solution for the needs of FreeType.

On 19/03/2018 19:00:48, Nikolaus Waxweiler <madig...@gmail.com> wrote:
I'm not familiar with graphics testing, but maybe algorithms exist that look 
for a magnitude of a difference between two images (maybe something like this 
exists in video encoding land?). Then you could ignore changes less than 1% or 
something.
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