[Mp4-tech] Regarding padding in MPEG-4 part 2

[Gary Sullivan]

Rickard et al,
For a long time, I was pretty sure that I knew what the answer was, and my interpretation agreed with yours.  Certainly that is the way the similar feature works in H.263 (although that fact is not directly relevant to the question at hand, since "motion vectors over picture boundaries" is a non-Baseline feature of H.263 Annex D, while MPEG-4 part 2 only tries to be compatible with the Baseline).  I vaguely recall that at one time one implementation of the reference software was doing it one way and the other was doing it the other way, and I believe MPEG eventually approved a corrigendum to Part 2 and a bug fix to the software to clarify it.  Unfortunately, I believe the clarification was according to the other interpretation.
There was a corrigendum finalized in 2004, which I think corresponded to MPEG output document N6362.  I believe this subject was addressed in that corrigendum.
If I was making an encoder, I might design it to avoid motion vectors reaching beyond the bottom and right edges of the reference pictures to be sure that I would work with decoders that used either interpretation.  
I guess another decent encoder approach would be to use padding in the source for those areas before encoding too, so that the only difference between the two interpretations would be the quantization error.  With that approach, a little drift might not produce very bad artifacts.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> Rickard Sjöberg (KI/EAB)
+> Sent: Wednesday, November 22, 2023 5:36 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] Regarding padding in MPEG-4 part 2
+> 
+> 
+> Dear experts,
+> 
+> assume that a simple profile video stream with height and 
+> width of 120 and 170 pixels respectively shall be decoded. 
+> The bounding rectangle of the reference VOP is 128x176. Now 
+> my question is whether you should pad outside of 120x170 or 
+> 128x176 when referencing pixels for motion compensation?
+> 
+> The relevant part of the standard is section 7.6.4 i believe 
+> (I looking at the 3rd edition, that's ISO/IEC 14496-2:2004, N5515):
+> 
+> The coordinates of a reference sample in the reference VOP, 
+> (yref, xref) is determined as follows :
+> xref = MIN ( MAX (xcurr+dx, vhmcsr), xdim+vhmcsr-1 )
+> yref = MIN ( MAX (ycurr+dy, vvmcsr), ydim+vvmcsr-1)
+> 
+> My interpretation of this is that padding should be done 
+> outside of 128x176 (this means that the pixels outside of 
+> 120x170 but inside of 128x176 shall be left as they were 
+> decoded), is this correct?
+> 
+> Is there any conformance bitstream that tests this behaviour?
+> 
+> /
+> Rickard Sjoberg
+> Ericsson
+> 
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