[FFmpeg-trac] #10495(avfilter:new): Optimized (x86 assembly) version of vf_ssim works incorrectly for some video inputs

[FFmpeg]

#10495: Optimized (x86 assembly) version of vf_ssim works incorrectly for some
video inputs
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             Reporter:  Evgeny    |                     Type:  defect
               Status:  new       |                 Priority:  normal
            Component:  avfilter  |                  Version:  git-master
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
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 Summary of the bug:

 Optimized assembly implementation of vf_sim is expected to behave
 identically to unoptimized (C-code) implementation, i.e. same metrics
 should be reported for the same input videos. However, due to bug in the
 assembly code, it’s not always the case, and the reported metrics can be
 different between the two. Optimized x86 assembly code works fine for
 1920x1080 input, but failed for 852x480 input.

 How to reproduce:
 {{{
 1. Build unoptimized ffmpeg binary by fully disable assembly optimization
 for entire ffmpeg
 % ./configure with --disable-asm
 2. Download test 1080p (8-bit) input video (e.g.
 https://media.xiph.org/video/derf/y4m/crowd_run_1080p50.y4m)
 3. Rescale 1080p video to 852x480p (8-bit):
 % ffmpeg -y -i crowd_run_1080p50.y4m -s 852x480 ref_852x480p50.y4m
 4. Generate degrade (8-bit) video using x264, e.g.:
 % ffmpeg -y -i ref_852x480p50.y4m -c:v libx264 -qp 40 x264_852x480p50.264
 5. Compare (8-bit) SSIM Y/U/V generated with unoptimized ffmpeg binary
 against SSIM generated using optimized ffmpeg binary:
 Sample command line:
 % ffmpeg -vsync 0 -i ref_852x480p50.y4m -i x264_852x480p50.264 -lavfi
 "ssim" -f null –
 Here is a difference between outputs for x86 assembly and C
 implementations:
 Optimized SSIM output: SSIM Y:0.770184 (6.386200) U:0.830635 (7.711759)
 V:0.845416 (8.108361) All:0.792798 (6.836059)
 Reference(Unoptimized) : SSIM Y:0.770184 (6.386200) U:0.829381 (7.679738)
 V:0.844320 (8.077677) All:0.792406 (6.827860)
 Some very tiny fractional difference could be observed due to compiler
 uses different floating point instructions. However, the difference is
 expected to be very small (e.g. < 1e-5).
 }}}
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Ticket URL: <https://trac.ffmpeg.org/ticket/10495>
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