x265, crf, slow, and file size

[strudo76]

I have some confusion I hope someone can help me out with.

I'm doing some research into what crf values to use for some video I'm encoding. Most articles say that slow offers the best quality to time ratio at a given crf value, and the official docs say to use whatever speed I can stand.

My understanding is that a given crf value indicates a constant quality setting. So why does the speed I choose affect the quality as well? Shouldn't a crf of 22 at slow give the same quality video as crf 22 at fast, just with a much larger file size?

I'm more interested in getting them done with good quality faster and not so concerned with the file size, though obviously I want them to be smaller than the source files.

So I'm asking you people, will the speed preset I use affect the quality of the final video, or is that solely determined by the crf value?

Thanks

[davexnet]

See this for some relevant info
https://kokomins.wordpress.com/2019/10/10/anime-encoding-guide-for-x265-and-why-to-nev...t-should-i-use

[Selur]

My understanding is that a given crf value indicates a constant quality setting. So why does the speed I choose affect the quality as well? Shouldn't a crf of 22 at slow give the same quality video as crf 22 at fast, just with a much larger file size?

crf is no objective quality measurement, it seems like your understanding of what crf (= constant rate factor) does is wrong.

Constant rate factor (CRF) is an encoding mode that adjusts the file data rate up or down to achieve a selected 'quality' level rather than a specific data rate based on the rest of the settings. So if you change anything aside from the the 'quality' of the output will change. Also note that since this 'quality' is for a specific source, a specific crf might look good for one source but might be not so good for another source. (hope this helps and doesn't cause even more confusion)

Cu Selur

[strudo76]

Thanks for the responses people. I see where I've got my wires crossed now. Thanks for that link too davexnet, very enlightening.