[akupenguin]

@LoRd_MuldeR
Hmm, sorta. Which variable is more fundamental depends on how you look at it.
CRF definitely treats RF as more fundamental: it sets RF to a constant, and doesn't ever compute anything related to bits unless you also enabled VBV.
You could consider 2pass to be using the CRF formula as just a relative allocation of bits and then multiply it by whatever scaling factor is needed. But you could also consider it to be using the CRF formula as a relative allocation of QP and then add whatever scaling factor is needed. Which is in turn equivalent to treating the RF parameter as the independent variable and bits as the dependent variable, and setting RF to whatever it needs to be. (Actually, the 2nd and 3rd views are slightly more accurate than the 1st view, since x264's "predict bits from QP" formula is not exactly an exponential. Adding QP is exactly what 2pass does, while multiplying bits is only an approximation of the effect.)
And whichever interpretation of 2pass you use, there's an analogous interpretation of ABR that's reevaluated at each frame based on the information available so far.

Btw, "final rate factor" is the final estimate of what RF would have produced the target bitrate if passed to a CRF encode. Not the actual RF used for the last frame, which is that plus a correction factor to compensate for the fact that ABR probably used the wrong RF earlier in the movie when it didn't know the complexity of the end. (Unless you set --ratetol=infinity, which disables that correction factor. In that case, the RF used for any given frame is just the best estimate so far of what RF would have produced the target bitrate if passed to a CRF encode, even if that means the current encode won't hit the target due to earlier mispredictions.)
... I can see how that would be confusing. Should we rename it?