[Mp4-tech] [Video][H.264]Regarding reference picture marking

[Alexis Michael Tourapis]

Dear Joseph,
This situation can happen when all references for a B slice are in the same
direction (either forward or backward). i.e. simple example: 
I0-P1-P2-P3-B4.... in this case L0 would be {3, 2, 1,0) while L1 is made
into {2, 3, 1, 0} based on this rule.
This was basically considered in an attempt to exploit references in a more
efficient way. Note that if this was not considered, you would most likely
be penalizing the encoding process especially for the bipredictive case.
Since the reference index is entropy coded, if no switching was performed
biprediction may had to select a different reference for one of its lists
and waste bits in coding the reference index (it is however possible to
still perform biprediction using the same reference and even using different
motion vectors and still get some advantage.). How likely do you think is
getting a biprediction from references {N,N} (i.e. 3 and 3 in the above
example)? Instead 3 and 2 are more likely to help prediction, and therefore
you would want the bit cost to be as small as possible. Making the lists a
bit more diverse can help in terms of how you exploit your lists and
reference indices. Even for single list prediction you may notice that
L1(ref0) and L0(ref0) would have also been identical and again maybe you are
wasting something in terms of prediction (and bits) since the information
you can find in one list can also be found in the other with similar (not
usually same however) cost. 
Not sure if this is clear enough.
Best regards,
Alexis
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Smitha Joseph
Sent: Tuesday, April 12, 2024 1:47 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] [Video][H.264]Regarding reference picture marking
Hi,
I had a doubt regarding the initialisation process for reference
picture lists for B slices in frames (8.2.4.2.3). The process
specifies that if reference list 1 has more than one entry and
reference list 0 and reference list 1 are identical, the first two
entries should be switched. Since the lists are identical why should
there be a switching? Could you please explain the reason behind this?
Thanks,
Smitha Joseph
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