[FFmpeg-trac] #10924(avfilter:new): Clean-up "ssim", "psnr" filter output

[FFmpeg]

#10924: Clean-up "ssim", "psnr" filter output
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             Reporter:               |                     Type:
  MasterQuestionable                 |  enhancement
               Status:  new          |                 Priority:  normal
            Component:  avfilter     |                  Version:
                                     |  unspecified
             Keywords:  ssim psnr    |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Related:
         https://trac.ffmpeg.org/ticket/10922#comment:4
 https://github.com/webmproject/libwebp/commit/33357131698ba0abb5b7a32b6f274f6f58170642#commitcomment-139962312

         I noted unnecessary verbosity in the info output:
         |1| Excessive trailing 0 of decimal.
         |2| "All:1 (inf)" alike already implied the same for each
 component. (pointless repeat)

         Misleading part of "psnr":
         |1| "min", "max" seem to just repeat "average"?
         |2| "average" would be better renamed to "all": for consistency,
 and it's really not the average of each component's sum.
         |3| Probably mostly relevant to WebP only: the "a" (alpha)..?
 (notably when the WebP has dummy alpha)

         And "ssim" seems missing support for Alpha? (per
 [https://ffmpeg.org/ffmpeg-filters.html#ssim the doc])

         With dummy input alpha (as in WebP), would it make sense to
 unconditionally ignore this channel?

         Disclaimer:
         I indeed don't quite understand these metrics.
         Which I used mostly only for telling whether 2 images are
 completely the same...
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Ticket URL: <https://trac.ffmpeg.org/ticket/10924>
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