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21st September 2018, 16:16 | #1 | Link |
Herr
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Which is a good image-quality metric?
Hi, I wonder, which metric is good for measuring image-quality?
I've seen some people often use "Avg QP" as image-quality metric (with low Avg QP as better quality), but isn't SSIM a better metric than that? Yeah, I know the eyes are the best image-quality thing to use, if you got good eye-sight. Thanks in advance. Last edited by Forteen88; 21st September 2018 at 16:25. |
21st September 2018, 16:19 | #2 | Link |
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Avg QP is technically not a quality metric, but a compression metric. There is a relation to quality of course, but its not its primary purpose.
Metrics like SSIM are far better for that. Or Netflix' VMAF.
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21st September 2018, 16:49 | #3 | Link |
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For comparison the PSNR of luma component (Y) only is quite fast and not bad. The x-y plot of SSIM (Y) to PSNR (Y) doesn't have much outliers and they have some strong positive correlation. In addition, it's great on detecting minor image shift which eyes cannot differentiate.
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23rd September 2018, 19:45 | #4 | Link |
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If your purpose is to assess the quality, with minimal changes from the original (visually lossless (equivalent jpeg qualities 90+))
butteraugli - https://github.com/google/butteraugli Other metrics of quality measurement will lie, because of the application of psychovisual algorithms in modern codecs. Last edited by zub35; 24th September 2018 at 00:38. |
26th September 2018, 00:47 | #5 | Link | |
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Quote:
VMAF is the least-bad metric we have today, but it is still limited by being a per-frame metric, which then gets averaged across frames. So an encode with frames ranging from 70-80 and an encode with frames ranging from 50-100 would both average a VMAF of 75. But the one going from 50-100 is going to have a lot more poor looking frames, and would deliver a much poorer subjective experience. VMAF is also calculated relative so source frame sizes, so the same VMAF score from a 720p source and a 1080p source will mean quite different things. The "right" way to do it would be to scale source and encodes up to 1080p to run the VMAF comparisons. But that really increases the calculation time for VMAF. |
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2nd October 2018, 03:42 | #7 | Link |
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Because no one has checked in code to do it ? VMAF elementary metric calculations should be MUCH faster to do in x265 than ffmpeg because the source and output frames are both right there in the CPU cache together. Since the VMAF model itself changes, actually calculating VMAF itself wouldn’t be as valuable as calculating the elementary metrics, which would allow improving VMAF accuracy for existing encodes whenever the machine learning model is updated. |
17th October 2018, 22:47 | #8 | Link | |
Herr
Join Date: Apr 2009
Location: North Europe
Posts: 556
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By the way, I read that VQM is also a good quality-metric,
Quote:
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