[Mp4-tech] [H.264] purpose of non-existing pictures

[Gary Sullivan]

Martin et al,
One type of use of this feature is to enable a form of temporal
bitstream scalability in which some reference pictures can be removed
from the bitstream without harming the ability to decode the remaining
pictures.  For example, I suggest reading the information in the
standard relating to "sub-sequence" concepts (esp. D.2.11 through
D.2.13).  I believe this is the primary purpose.
Also, it can provide a form of robustness to transmission-channel losses
of some reference pictures.  If a reference picture becomes
"non-existing" due to packet losses, the remaining pictures in the
bitstream may still be decodable.  This definition provides a
well-defined decoder behavior for environments in which in the system
operation may result in the loss of some reference pictures.  For such a
case, it might not really be necessary to specify how the decoder would
respond to a picture loss -- however, this feature allows the encoder to
decide whether the decoder should flag the loss of a picture as a real
problem or not (by setting the value of the
gaps_in_frame_num_value_allowed_flag to 0 or 1) and allows the encoder
to understand how the decoder will respond to such events, which can be
beneficial to know for various purposes.
In fact, the way the decoder responds to a missing reference picture
when gaps_in_frame_num_value_allowed_flag is equal to 1 may often also
be a good way for the decoder to respond to a missing reference picture
when gaps_in_frame_num_value_allowed_flag is equal to 0 as well.  But
when when gaps_in_frame_num_value_allowed_flag is equal to 0, it is up
to the individual product designer to decide how the decoder should
respond to missing reference pictures.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> Martin.Lange sci-worx.com
+> Sent: Wednesday, May 31, 2024 7:14 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] [H.264] purpose of non-existing pictures
+> 
+> Hello everyone,
+> 
+> a thing which I was always wondering is the purpose of non-existing
+> pictures. What is a scenario in which an encoder would decide to
+> intentionally introduce gaps in frame_num, thus producing 
+> non-existing
+> pictures? The only effect I see is that they make other pictures fall
+> out of the decoded picture buffer and that they also get incorporated
+> into reference lists, thus changing the reference indices 
+> other pictures
+> would have without their presence. Can this lead to any gain 
+> in coding
+> efficiency? Can anybody comment on what was the intention of 
+> this tool?
+> 
+> If this is a question which has already been posted in the 
+> past, I would
+> be happy if anyone could point me to some kind of FAQ of 
+> this forum, if
+> it exists (does it?).
+> 
+> Best regards,
+> Martin Lange
+> 
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