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Old 29th August 2020, 16:00   #1  |  Link
der_groschi
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Join Date: Sep 2007
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How can i make x264 allocate more bits to "clean", fewer to grainy/noisy sources?

I've been encoding my movies in CRF mode, 720p resolution for a long time since that was all my aging home cinema was capable of anyway. Then i eventually moved to a smaller place, replaced the projector with a UHD TV and switched my new encodes to 1080p.With the higher bitrates needed for that (as well as the higher contrast of modern TV displays vs projections, making any problem more obvious), i'm a bit more concerned about overall encoding efficiency now than before.

I've noticed that - in my eyes at least, as i'm well aware that different people might be bothered the most by different kinds of encoding artifacts or the places/circumstances in which they occur - most digitally shot, clean-looking films could use a lower CRF value since i'm quite sensitive to artifacts in flat areas, static shots.

Also i found that i could get away with somewhat higher CRF values for Grainy/Noisy material... i guess that's just how the large amount of (pseudo-)detail of film grain is able to mask quite a few inconsistencies that would be more bothersome without it. Kinda like some dithering after the fact can obscure some amount of banding or blocking.

So i tried to find a new "set and forget" solution that reaches results to my satisfaction on most films without wasting too much space.

First i tried tweaking --qcomp, which did a little but not enough to reach my goal of reducing the bitrate-gulf between grainy vs. clean sources. As tweaking that setting too much has been discouraged a lot anyway, i didn't bother further with that.

I found that switching from --aq-mode 1 to mode 2 or 3 (that "dark scenes"-thing with mode 3 also comes in handy for me) also brings me a bit closer to my goal.

Then i tried tweaking the --aq-strength which - if i understand this correctly - is a lot about tweaking the balance of allocating bits to high- vs. low-complexity areas/scenes. Bitrate-wise, increasing the value to something in the area of 1.5 - 2.0 seemed to do the job bitrate-wise, bringing down the bitrate for grainy sources while still looking good to me. Unfortunately for very clean sources - despite some increase in overall bitrate - it introduced ringing and temporal blurring problems around edges. Well, that's kind of the nature of what AQ does at higher values, i guess.

It seems like a simple task: Bring the bitrates of high-complexity vs. low-complexity sources quite a bit closer together. But i'm a bit stumped by now. Does anyone have some other suggestions how i could achieve the desired balance without having to manually tweak CRF for every movie?

Last edited by der_groschi; 29th August 2020 at 16:03.
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