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

[MPEGIF List Admins]

Another forward. 
(Herbert - please subscribe with your sending address 
 or send with the subscribed address ...
 Thanks.) 
-----Original Message-----
From: Herbert Thoma [mailto:tma iis.fhg.de] 
Sent: Thursday, 23 November 2023 12:51
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 Sullivan wrote:
> Have you looked at N6362?

I just did. The only item about padding I see is
"In subclause 7.6.4, clarify the padding process in case of interlaced
sequences"
We keep the sentence: "... A bounding rectangle is defined by vop_width and
vop_height extended to multiple of 16 ..."
And in ISO/IEC 14496-2:2004 subclause 7.6.4 states later:
"... Note that for a rectangular VOP, a reference VOP is defined by
video_object_layer_width and video_object_layer_height, extended to
a multiple of 16 ..."
So I don't think that we changed anything regarding padding with N6362.
Things were so easy with H.263: you got SQCIF, QCIF, CIF, 4CIF and 16CIF.
That's it, and all are multiples of 16. With H.263+ we got custom picture
formats ...
Kind Regards,
  Herbert.
> 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
> +> > +> 
> +> > +> _______________________________________________
> +> > +> NOTE: Please use clear subject lines for your posts. Include 
> +> > +> [audio, [video], [systems], [general] or another 
> +> > +> apppropriate identifier to indicate the type of 
> +> question you have.
> +> > +> 
> +> > +> Note: Conduct on the mailing list is subject to the 
> +> > +> Antitrust guidelines found at 
> +> > +> http://www.mpegif.org/public/documents/vault/mp-out-30042-Ant
> +> > +> itrust.php
> +> > +> 
> +> > 
> +> > _______________________________________________
> +> > NOTE: Please use clear subject lines for your posts. 
> +> Include [audio, [video], [systems], [general] or another 
> +> apppropriate identifier to indicate the type of question you have.
> +> > 
> +> > Note: Conduct on the mailing list is subject to the 
> +> Antitrust guidelines found at 
> +> http://www.mpegif.org/public/documents/vault/mp-out-30042-Ant
> +> itrust.php
> +> > 
> +> 
> +> -- 
> +> 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/
> +> 
> 

-- 
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/