x264 question (CRF18 + presets)

[samtroy]

Hi all, quick question:

If I encode a movie

1) using CRF 18 and 'medium' preset

and

2) using CRF 18 and 'faster' preset (which is faster of course)

will the quality be *exactly* the same and only the filesize will be larger?

Thanks in advance, S.

[sneaker_ger]

No, quality will not be exactly the same. Constant CRF does not guarantee constant quality for different settings.

[samtroy]

Thanks for your answer.

OK, but isn't constant quality the whole point in CRF encoding? And would the quality difference be noticable or is it only theoretical?

[Dark Shikari]

Constant quality means "constant throughout the video, by x264's measure of quality". It does not mean "constant regardless of what settings you use, the laws of physics in the universe you're in, or whether or not your glasses are on."

Since different settings may change x264's definition of quality -- slower settings will use a better one -- x264 can't possibly guarantee identical quality across all settings.

[samtroy]

I see. Thanks for the explanation.