Improve SSIM implementation

[paulirish]

I was interested in how the SSIM we consume (image-ssim) is different than that of the reference implementation.

visualmetrics uses https://github.com/jterrace/pyssim which points to https://ece.uwaterloo.ca/~z70wang/research/ssim/ .. the reference table there with einstein has the last SSIM at 0.662
However our module reports (via http://darosh.github.io/image-ssim-js/test/browser_test.html) this pair as 0.741.
Also the implementation looks fairly different than most of the other SSIM modules I'm looking at.

There are two new modules for SSIM, I'm seeing: https://github.com/obartra/ssim and https://github.com/IonicaBizau/img-ssim

However, both of them are unattractive in their current state.

Regardless, I'd like to still keep this issue open to explore changing our SSIM dependency.

[pahammad]

This is a very interesting/valid question. How could we evaluate or compare these different SSIM implementations? Looking at 1-2 videos isn't sufficiently convincing per se (choice of webpages and sampling issues will complicate things).

[pahammad]

One potential idea would be to take all the WPT videos from SpeedPerception study, and compute all different variations and see which variation explains the human judgment the best. Such a comparison makes a lot more sense using the Phase-2 data (that's currently live) because we included both Alexa + InternetRetailer top-K lists along with both mobile and desktop rendering modes for comparisons. In Phase-1 deep-dive analysis, we already compared several different metrics - so adding different variations of SSIM into the mix is fairly easy to do. Let me know if you see any problems with this line of thought.