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

[Gary Sullivan]

Bita et al,
If you're working on sequences that include fades, it might be a good
idea to try using weighted prediction.
Best Regards,
Gary Sullivan
________________________________
	From: jvt-experts-bounces lists.rwth-aachen.de
[mailto:jvt-experts-bounces lists.rwth-aachen.de] On Behalf Of Tourapis,
Alexis
	Sent: Sunday, November 19, 2023 12:33 PM
	To: bita damghanian; jvt-experts lists.rwth-aachen.de
	Cc: mp4-tech lists.mpegif.org
	Subject: Re: [jvt-experts] why B frames bitrates must be smaller
than P framesbitrates?
	Dear Bita,
	The Lagrangian costs for motion estimation and Rate Distortion
Optimized mode decision are also different (higher) by default in the
reference software. Their values tend to give higher priority to inter
modes and in particular skip since they tend to give a preference to
modes that result in fewer bits instead of lower distortion. The general
assumption (right or wrong) is that for B slices you can sacrifice a bit
more in terms of quality and have savings on bitrate which would
translate to bigger savings and smaller impact in quality on the overall
sequence. You can change the lagrangian costs either by modifying the
software or by using the explicit lagrangian parameters in the
configuration file (UseExplicitLambdaParams)
	Furthermore you may wish to set the following flags 
	DisableThresholding = 1
	DisableBSkipRDO = 1
	The second one can be especially important since it avoids
forcing the test of skip modes in B slices (normally the encoder will
test if the motion vectors corresponding to skip/direct also result in a
coded residual and based on that determine if mode is skip or direct).
	Although bipredictive ME could improve the chances of getting
skip/direct modes and of course bi-predictive partitions, I do not
understand your reasoning of disabling this. Are you trying to bias
intra modes during fading transitions? Furthermore, I am also assuming
that you are not using weighted prediction which was introduced in the
standard for this particular reason.
	Finally, I am assuming you are talking about the encoder.cfg
file in terms of defined QP values for P and B slices. The reference
software does not specify any such condition in general.
	Best regards,
	Alexis
________________________________
	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 12:02 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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