[Mp4-tech] question about ref picture lists in H.264

[Alexis Michael Tourapis]

Dear Narendra,
There is only one reference buffer. The two lists basically specify the
mapping of the references within this buffer. Nothing to do with directions.
For reference, B slices do not need to use references coming always from
both directions (i.e. all references coming from the past or the future),
while it is even possible for P slices to have references from both
directions. Now having two lists basically provides you with certain
flexibility especially in terms of how you order your references, code them,
or allocate weights (a very important feature of H.264).
Best regards,
Alexis
  _____  
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Narendra Joshi
Sent: Thursday, April 21, 2024 3:09 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] question about ref picture lists in H.264
Hello there,
Two reference picture lists are specified in H.264, ref picture list 0 and
list 1.
For P slices, only ref pict list 0 is used; and for B, both list 0 and 1 are
used.
It's not clear to me why 2 separate ref pic lists are maintained ?
Can't we just use one single list, and then use 1 index pointing to ref pic
for P slice 
and 2 indices for B slice ?
Is it just because B slice needs future pictures as well ? (But still then,
can't it be managed
just with a single list ?)
Am I missing something ?  Please reply,
Thanks in advance,
Narendra
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