regtest threshold problem

[Werner LEMBERG]

I'm working on a modification of `musicxml2ly` that will cause a
change in the result as shown by the attached images.  However, our
regtest classifies this as 'below threshold' and doesn't display it.

Does anybody have an idea how to improve this?  I'm not an expert in
image analysis, and a quick search in the internet didn't help me in
finding a good metrics.  I think we would need something that checks
whether a shape completely disappears and/or reappears in the images
(contrary to shapes that are only slightly shifted and/or distorted),
almost regardless of its size.


    Werner