Why not print crf after 2pass ABR? only, difference, determine, explained

andy

Revision 371 of x264 is labelled as
after a 1pass ABR, print the value of --crf which would result in the same bitrate.

This seems useful. However, I was wondering why the same is not done after 2pass ABR?

crf option can be used only on 1pass ABR.

Maybe what I'm asking is explained by this quote:

Originally Posted by akupenguinABR and 2pass use the same rc_eq, the primary difference is in how they determine rate_factor. In ABR, we only have statistics about the past frames, so we continually update the estimate of the rate_factor needed to meet the target bitrate. In 2pass, we have all the stats, so we pick the right rate_factor to begin with.

So the second pass has simply chosen a rate_factor (which is what can be specified by the --crf option) that will give the target bitrate. I don't understand why this rate_factor is not printed.

2nd pass rate_factor is in entirely different units. CRF's rate_factor maps (SATD from ME prepass) to QP. 2nd pass's rate_factor maps (1st pass's bits) to QP. I give them the same name because they perform the same function, and both work because the mapping trains itself on your video. But if I just printed the number used internally for the 2nd pass, it would not give any sort of predictable bitrate for CRF.

Aah, ok, thanks for clearing that up.