x264 threads

[alkoon]

Good day everyone

I would like to know using more threads does effect the quality & of course without VBV?

like if I sued this commands

x264 --crf 17 --threads 6
x264 --crf 17 --threads 14

Thanx

[poisondeathray]

More threads adversely affects quality and compression ratio, but it's mostly negligible for threads 6 vs 14 at crf 17 .

It's measurable in metrics, and the filesize will be slightly different so you 'd have to so serial tests at 17.x until the bitrates match, but at high CRF , low bitrate ranges high amount of threads can become more important and noticable

VBV also adversely affects quality and compression ratio, and CRF+VBV sometimes has bizarre artifacts, not recommended

[Selur]

CRF+VBV sometimes has bizarre artifacts, not recommended

Can you elaborate on that ? I haven't had issues with crf and vbv for years,... (at least no issues I could spot; no unexpected artifacts or vbv violations,..)

Cu Selur

[poisondeathray]

CRF+VBV sometimes has bizarre artifacts, not recommended

Can you elaborate on that ? I haven't had issues with crf and vbv for years,... (at least no issues I could spot; no unexpected artifacts or vbv violations,..)

It occurs usually around scene changes, the quality of the affected frame is usually dramatically lower or noticably different than other frames

[alkoon]

Thank you for this information
Actually I'm surprised that with more threads I got more bitrate and more size!!

[LigH.de]

No surprise. More threads means that the frame content is split into smaller pieces, and each thread "sees" less video content to compare with and to find similarities useful to spare size. The result will have more intra blocks it could not find a smaller substitution for.

It is not recommended to use more threads than the encoder heuristic calculates. More threads will only provide a small amount of speed-up due to saturation in the multi-threading management, but the encoding will be less efficient per thread.