AV1 vs VP9 vs AVC (h.264) vs HEVC (h.265): Part III - Quality

Visual quality is somewhat subjective. However, Efforts are still being made to quantify it. There is a perceptual video quality assessment algorithm developed by Netflix called VMAF - Video Multi-Method Assessment Fusion.

We tried to tune different CRF values to encode the video clips we used in previous parts. But unfortunately, we found that decreasing the CRF did not increase the VMAF score, it even dropped a little bit. We are not able to do a quality comparison at this moment.

Anyway, a feature of x264 called Psy RDO caught our attention:

the human eye doesn't just want the image to look similar to the original, it wants the image to have similar complexity. Therefore, we would rather see a somewhat distorted but still detailed block than a non-distorted but completely blurred block. The result is a bias towards a detailed and/or grainy output image, a bit like xvid except that its actual detail rather than ugly blocking.

This preference is quite easy to understand, people wear glasses for more detailed, less blurry vision even though the vision is a little bit distorted. Besides, camera lenses, by its nature, make more or less distorted images anyway.

Image by 955169, licensed under CC0.

Mainly, videos are shot either for archiving or for showing to people. If a video is for archiving, the more similar to the original, the better. If it is for showing, the more pleasing to eyes, the better.

Transcoding for saving disk space is generally not a good idea, as video compression technologies are still involving, there may be better algorithms coming out in the future. Transcoding is lossy, it brings artifacts and these artifacts will make things worse when the video is transcoded again and again. But if you still want to do it, you should avoid codec features that distort the video intendedly. Most of these features are enabled by default.

Still, we should judge video quality with our own eyes.



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