x264 presets and CRF

[iSeries]

Hi,

If I encoded the same movie twice, once at CRF20 with --preset medium, and then at CRF20 with --preset slow, aside from any file size differences would the encodes have the same visual quality?

Thanks!

[nm]

If I encoded the same movie twice, once at CRF20 with --preset medium, and then at CRF20 with --preset slow, aside from any file size differences would the encodes have the same visual quality?

No. Only roughly the same. The quality/bitrate ratio is higher with --preset slow, but you might get a smaller file and the same quality or a larger file and better quality, for example.

[nurbs]

You might also get a smaller file with lower quality, but the quality drop would be less than the size drop.
If you compare the defaults with faster or veryfast at the same CRF you often get a massive filesize drop combined with an even greater quality drop because you turned off psy.

CRF isn't really constant quality. It's only roughly constant for the same settings with similar sources.

[Blue_MiSfit]

Indeed.

CRF is very handy dandy when you don't really care about hitting a SPECIFIC file size, but it's definitely not "really" constant quality.

I use it all the time, but when evaluating movie to movie, I usually do testing at a fixed bitrate, using 2 pass... Of course, seeing final bitrates / visual quality from matched CRF across different movies is a lot of fun too :devil:

Derek