[FFmpeg-user] Two-pass versus CRF: a curiosity question

[Nicolas George]

Le primidi 21 brumaire, an CCXXV, Fred M. Sloniker a écrit :
> I'm given to understand that, under the hood, two-pass and CRF are the same
> thing, just that two-pass calculates the necessary CRF to get the desired
> filesize. Is this correct?

I do not think it works exactly that way. For starters, it depends on
the encoder: CRF are features of x264 and x265, they may not work
exactly the same way.

But the rule of thumb is: two-pass tries to average the quality at a
given bitrate to have it as constant as possible, while CRF does
constant quality right from the start.

>			     And if so, is it possible to see what CRF was
> selected in the two-pass encoding?

I do not know how.

>				     I'm experimenting with encoding two
> versions of a video, one at a higher resolution than the other, and I'm
> curious if I could encode the high-res one with two-pass, then somehow
> encode the low-res one with the same CRF.

Do not do that, it makes no sense.

For starters, CRF is a local quality measurement. Using the same CRF
with a lower resolution will result in lower overall quality. The scale
of CRF - subjective quality is different for all resolution.

Use the settings that make sense for your needs. What are the
constraints that govern your choice of quality level?

Regards,

-- 
  Nicolas George
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