For our purposes of quality assurance in GES, we want to have a working video stream comparator, in order to spot regressions and to ensure frame-accurate editing.

SSIM fits the bill, but it is very difficult to port in a standard way.

My approach for fixing it has thus been to extract a base class from videomixer, and only keep the relevant video measuring code in videomeasure_ssim.c itself. 

It works well, and thanks to the colorspace conversions capabilities in basemixer its usage is now much more straightforward. Additionnally, it will be relatively easy to make ssim compare n-streams in one pass instead of just two for now, and that will also be very useful performance-wise for us.

Example usage is :

gst-launch-1.0 -m videotestsrc num-buffers=30 ! ssim name=m ! videoconvert ! xvimagesink sync=false videotestsrc num-buffers=30 ! m.

As a bonus it shows that usage of the smpte pattern in videotestsrc is not deterministic (Might not be very relevant though :)

I have also ported videomixer to that base class and have been using it since multiple days, with no behaviour difference.

In the future, I think having that base class would be useful, for example when comes the time of implementing a glmixer, but that's not critical.

Patches follow.