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

[Gary Sullivan]

Have you looked at N6362?
Best Regards,
-Gary Sullivan
+> -----Original Message-----
+> From: Herbert Thoma [mailto:herbert.thoma iis.fraunhofer.de] 
+> Sent: Thursday, November 23, 2023 2:11 AM
+> To: Gary Sullivan
+> Cc: "Rickard Sjöberg (KI/EAB)"; mp4-tech lists.mpegif.org
+> Subject: Re: [Mp4-tech] Regarding padding in MPEG-4 part 2
+> 
+> Gary, Rickard,
+> 
+> I am pretty sure that the padding from 128x176 is the 
+> correct interpretation
+> (meaning that the pixels outside of 120x170 but inside of 
+> 128x176 shall be left
+> as they were decoded).
+> 
+> I remember the discussion in MPEG very well, because I 
+> originally implemented
+> it the other way in my encoder and decoder and changed that 
+> after the corrigendum.
+> 
+> I don't konw if there are any comformance bitstreams 
+> available, but I attached
+> a few frames of forman cropped to 170x120 and encoded with 
+> my encoder. (I can
+> not guarantee that there are actually motion vectors that 
+> test the problem
+> in there, though.)
+> 
+> Kind regards,
+>   Herbert.
+> 
+> Gary Sullivan wrote:
+> > 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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+> 
+> -- 
+> Herbert Thoma
+> Head of Video Group
+> Multimedia Realtime Systems Department
+> Fraunhofer IIS
+> Am Wolfsmantel 33, 91058 Erlangen, Germany
+> Phone: +49-9131-776-323
+> Fax:   +49-9131-776-399
+> email: tma iis.fhg.de
+> www: http://www.iis.fhg.de/
+>