Quote:
Originally Posted by imcold
It's inherently worse quality-wise only when you do slice-based threading, where you lose compression efficiency at slice boundaries, otherwise it depends? I don't remember the specifics for x264 that much, but iirc with default frame-based threading (1 thread per frame), only the vertical motion search for following frame is restricted. With high enough vertical video resolution, that gets irrelevant, because levels give restrictions on the vertical mv range anyway (unless you throw level compliance out of the window, ofc).
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Right, there are features that improve efficiency without impacting quality materially, or at all.
For example --b-adapt 1 is now effectively the same quality as --b-adapt 2, although IIRC the defaults haven't been updated accordingly. Compiler optimization like specific CPU targeting or profile driven optimization improve perf some without any quality impact.
x265 has a whole bunch of other features that improve parallelization without impacting quality, some of which could probably be backported to x264. For example, parallel mode estimation (--pmode). It can improve quality a bit AND increases parallelization (although the fps/thread gets a lot worse).