why are divx b-frames at best quant4?

[draggoon01]

is there a technical reason?

[gldblade]

DivX B-frames have quantizers double that of normal P and I-frames. Because the lowest quantizer that P and I-frames can have is 2, B-frames' lowest quantizer is 4.

[draggoon01]

thanks for the anwer.

is that doubling a requirement in the mpeg4 spec?

[gldblade]

No. It depends on the codec. In XviD, for example, B-frame quantizers can be an average of the quantizers of the frames before and after the B-frame, depending on your settings.