[Mp4-tech] [H.264 Video] Question about decoded frames ordering foroutput

[Gary Sullivan]

The answer to your first question is that Yes, it is possible for some
picture(s) that follow an IDR picture in decoding order to precede the
IDR picture in output order.  This happens whenever the POC of a picture
has a negative value.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> Alejandro Moya
+> Sent: Wednesday, August 03, 2023 2:36 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] [H.264 Video] Question about decoded 
+> frames ordering foroutput
+> 
+> Hi again, everyone!
+> 
+> I have a couple of doubts about the use of POC in decoded frames.
+> 
+> First of all, when finished decoding an IDR frame, this one can be
+> outputted directly to the display or maybe there could be any later
+> frame in decoding order that should be send to the display 
+> before this
+> IDR frame? I mean, let's say this is the decoding order:
+> 
+> POCs ->    ... 24 -  21 -  20 - 19 - 25 -  0(IDR) - 2 - 26(Delayed
+> frame) - 4 - 3....
+> 
+> Could be this situation posible in any H.264 bitstream at 
+> any level or profile?
+> 
+> 
+> And the second question is about real-time applications of H.264.
+> Let's say the maximum allowed delay is 1 frame for some real-time
+> scenario... Is it a good idea to use POC Type 0 or 1 in this
+> situation? I think that the decoder should wait to fill all its DPB
+> before sending to the output any decoded frame, so this delay makes
+> POC Type 0 and 1 not suitable for real-time. Or maybe I am wrong? I
+> got to this conclusion analyzing sequences like this one:
+> 
+> POCs (POC Type = 0) ->    .... 0(IDR) - 2 - 4 - 6 - 3 - 1 - 5.....
+> 
+> The decoder can't display any frame till the one with POC 1 is
+> decoded, whereas in the following situation, the sequence is properly
+> ordered in decoding order so it could be displayed without any delay:
+> 
+> POCs (POC Type = 0)->     ..... 0(IDR) - 2 - 4 - 6 - 8 - 10 - 12.....
+> 
+> I suppose all sequences have only I and P slices.
+> 
+> Thank you all in advance!!
+> 
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