Allows B-frames to be used as references for predicting other
frames. For example, consider 3 consecutive B-frames: I0 B1 B2
B3 P4. Without this option, B-frames follow the same pattern as
MPEG-[124]. So they are coded in the order I0 P4 B1 B2 B3, and
all the B-frames are predicted from I0 and P4. With this op-
tion, they are coded as I0 P4 B2 B1 B3. B2 is the same as
above, but B1 is predicted from I0 and B2, and B3 is predicted
from B2 and P4. This usually results in slightly improved com-
pression, at almost no speed cost. However, this is an experi-
mental option: it is not fully tuned and may not always help.
Requires bframes >=2. Disadvantage: increases decoding delay
to 2 frames.
