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Old 8th August 2020, 08:23   #29832  |  Link
Sharc
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Join Date: May 2006
Posts: 3,997
Quote:
Originally Posted by jdobbs View Post
I ran numerous tests on numerous sources using that setting. Even though the encode slowed down significantly (making you think it does more), the SSIM results were worse than the next lower setting (quality, without multipass). All the existing settings position in the quality list were based on SSIM values first and encode speed second.

My results could be wrong... but it would have to be wrong on every source I tried.
I have some doubt whether NVIDIA's 2pass method contributes much to quality in the sense of best match with the original or highest SSIM. They write:
Quote:
As a result, with 2-pass rate control modes, NVENC can distribute the bits more optimally within the frame and can reach closer to the target bitrate, especially for CBR encoding. Note, however, that everything else being the same, performance of 2-pass rate control mode is lower than that of 1-pass rate control mode. The client application should choose an appropriate multi-pass rate control mode after evaluating various modes, as each of the modes has its own advantages and disadvantages.
In earlier tests I found that disabling -aq resulted in better SSIM than using it. The explanation is probably that -aq is to some extent a psychovisual optimization which conflicts with PSNR -- and possibly also with SSIM --optimization.
From NVIDIA on Spatial AQ:
Quote:
Spatial AQ mode adjusts the QP values based on spatial characteristics of the frame. Since the low complexity flat regions are visually more perceptible to quality differences than high complexity detailed regions, extra bits are allocated to flat regions of the frame at the cost of the regions having high spatial detail. Although spatial AQ improves the perceptible visual quality of the encoded video, the required bit redistribution results in PSNR drop in most of the cases. Therefore, during PSNR-based evaluation, this feature should be turned off.
Perhaps we see a similar effect with the 2pass modes?

Last edited by Sharc; 8th August 2020 at 08:33.
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