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Old 2nd August 2023, 14:52   #1  |  Link
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Low PSNR and high VMAF score

I am converting some 1080p movies to HEVC. Didn't see this with my 4k encodes but basically I extract the content of the video and put it into an MP4 file. Then open it in Premiere and run NeatVideo to clean and slightly sharpen the video. Then export it as DNxHR HQ.

After that I run thru FFMpeg with basically this command:

ffmpeg.exe -i <input>.mxf -c:v libx265 -x265-params level=51:high-tier=1:repeat-headers=1:crf=20:no-sao=1:info=0:range=limited:vbv-bufsize=17500:keyint=24 -preset slow -pix_fmt yuv420p -sn -an <output>.hevc
Run the original source file against the resulting HEVC file (in MKV container) and get:

PSNR: 37.9802
SSIM: 0.9745
VMAF: 96.8085

The VMAF score is good but why the PSNR score low? Think usually it's recommended to be in the 40's? Tried lowering the CRF to 18 but didn't change much.

Last edited by jriker1; 2nd August 2023 at 15:17.
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Old 2nd August 2023, 19:50   #2  |  Link
Join Date: Jan 2006
Location: Portland, OR
Posts: 4,817
38 isn't that low a PSNR score at all. Note that those three metrics are on quite different scales, so you can't really compare them in this form. SSIM (dB) is on the same scale as PSNR, but generally the same frame will have a substantially lower SSIM than PSNR.

In terms of perceptual correlation VMAF > SSIM > PSNR, so trust VMAF over PSNR (although even VMAF isn't that accurate). Frame QP is probably the most accurate in terms of predicting compression artifacts (and is the only decent one for HDR), and is on a fourth scale.

It might be helpful to step back and explain what you are trying to do with quality metrics, and we could make some more focused suggestions.
Ben Waggoner
Principal Video Specialist, Amazon Prime Video

My Compression Book
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ffmpeg, psnr, vmaf

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