[Mp4-tech] RE: [jvt-experts] why B frames bitrates must be smaller than P framesbitrates?

[Dzung Hoang]

One reason not mentioned is that the distance to the nearest reference frame
is larger for P frames than for B frames. Therefore what looks like a slow
fade to B frames looks like a quick fade to P frames.
Regards,
- Dzung Hoang
  _____  
From: jvt-experts-bounces lists.rwth-aachen.de
[mailto:jvt-experts-bounces lists.rwth-aachen.de] On Behalf Of bita
damghanian
Sent: Sunday, November 19, 2023 2:02 PM
To: jvt-experts lists.rwth-aachen.de
Cc: mp4-tech lists.mpegif.org
Subject: [jvt-experts] why B frames bitrates must be smaller than P
framesbitrates?
Dear all experts,
my experiments show that in fade out sequences(where the frames luminance
decrease in consequetive frames), the P frames macroblocks are mostly Intra
coded.
the case is also true for B frames.
but in long fades where the luminance decrease between two consequetive
frames is so small, the B frames MBs are mostly Skipped. but in this
situation the P frames macroblocks are still mostly Intra coded.   
I don't know why when P frames MBs are intra coded, the B frames MBs are
skip coded.  
I found 3 reasons for this:
1- Bi-predictive ME for B frames and being able to be predicted from
different directions, make B frames be coded more optimum.
    to  omit thit effect, I disabled Bi-predictive ME and defined 1 number
of reference frames for B frames.
2- In jm10.2 the QP defind for B frames are greater than the QP defind for P
frames and thus B frames are mostly prefer to be skipprd.
    to  omit thit effect, I defined equal QPs for P and B frames
3-rate control should be off
but still the problem remains:
what are the other factors which make B frames MBs to be more skipped rather
than to be Intra coded like P frames MBs?
in other words, why B frames bitrates must be smaller than P frames
bitrates? what are the parameters in the encoder which force this?
thanks in advance for your attention and help,
Bita Damghanian
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