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Old 29th April 2019, 16:32   #6792  |  Link
Boulder
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Join Date: Mar 2002
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Quote:
Originally Posted by Blue_MiSfit View Post
I'd caution against doing too much settings tuning using still frame comparisons, especially when evaluating max CU size (--ctu).

I've spent a lot of time doing this. Using --ctu 32 can indeed produce a sharper / more detailed / less blurred image when looking at single frames - relative to keeping the default (--ctu 64) in some cases. However, the trade-off is typically more blocking, which you're more likely to notice in motion. In other words, an inferior user experience unless you're using very high bitrates.

You're kind of performing a brute force non-adaptive psy tuning. x265 will use smaller CUs when it sees fit, you're just forcing it to never consider 64x64, even when coding giant uniform areas.

In my opinion, you're typically best off trusting the extensive tuning and testing that MulticoreWare has done, and just using a preset. They set --ctu to 64 starting with --preset veryfast, and keep it that way all the way through placebo. In fact, in looking at the documentation for this parameter: https://x265.readthedocs.io/en/defau...#cmdoption-ctu it appears the only reason they set this to 32 for superfast and lower is that this increases parallelism and can therefore be faster.

Do what's right for you, of course
I've always considered 64x64 to be quite a lot for 720p video, and also the difference in performance is substantial. I can see that the average QP is lower with 32x32 than 16x16 though so it could be that there are situations where the encoder could squeeze some more out of the video. Of course, I could always run two simultaneous encodes to keep the CPU as busy as possible.

As you apparently have been testing things quite a lot, do you have any opinion regarding AQ-mode? I'm still quite unsure if the new default of mode 2 is the best general option for film.
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