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I want to use ssim to evaluate the compression quality. However, when I calculate the ssim values I get very similar (and low) results for very good lossless compression and for very bad.

What do I do wrong ?

For compression I do:

ffmpeg -r 6 -y -i %005d.bmp+ -c:v libx264 -crf 1 x264_medium_crf_1.avi
ffmpeg -r 6 -y -i %005d.bmp+ -c:v libx264 -crf 51 x264_medium_crf_51.avi

Ref by -crf 0:

ffmpeg -r 6 -y -i %005d.bmp+ -c:v libx264 -crf 0 ref.avi

For ssim:

ffmpeg -i x264_medium_crf_1.avi -i ref.avi -lavfi "ssim;[0:v][1:v]psnr" -f null -
ffmpeg -i x264_medium_crf_51.avi -i ref.avi -lavfi "ssim;[0:v][1:v]psnr" -f null -

I get

[Parsed_ssim_0 @ 000002aa1f6c2c40] SSIM Y:0.891869 (9.660499) U:0.955877 (13.553365) V:0.907188 (10.323956) All:0.918311 (10.878384)
[Parsed_ssim_0 @ 00000230980ecf00] SSIM Y:0.883053 (9.320127) U:0.956858 (13.650996) V:0.897236 (9.881609) All:0.912383 (10.574098)
idoo
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  • Works for me. How did you create `ref.avi`? Are you sure the timebases are the same? – llogan Mar 04 '21 at 17:36
  • @llogan, I created ref.avi the same way I created the 2 other files but with "-crf 0" instead of "-crf 1" and "-crf 51" I edited my question with this explanation. – idoo Mar 07 '21 at 12:56

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