The mean YUV-SSIM value of all frames in a piece of content really isn't a very useful metric. It's better than Y-only PSNR (which we see a lot). But the mean of a bunch of frames doesn't account for variability in the quality throughout an encode. A file that oscillates between -5 dB and -15 dB has the same mean as a consistent -10 dB, but will look much worse.
A single value rating of the quality of a file of >>10 second is really an unsolved problem for the industry, and an essential one if we want to make broad comparisons like this.
Even just publishing the variance of the metrics would help a lot.
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Ben Waggoner
Principal Video Specialist, Amazon Prime Video
My Compression Book
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