H264EncoderParams has a rate_control.method allowing one to specify H264_RCM_VBR or H264_RCM_CBR. But there doesn't appear to be any way to set the two separate bit rates for VBR. How does one do that?
In addition to VBR bit rates, what other parameters affect compression efficiency? Presently, without setting any parameters, compression efficiency is terrible, producing MPEG-4 files some three or four times larger than codecs with the right parameters.