FW: [Mp4-tech] Regarding padding in MPEG-4 part 2
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another forward of a bounced email.
-----Original Message-----
From: Herbert Thoma [mailto:tma iis.fhg.de]
Sent: Thursday, 23 November 2023 12:59
To: Gary Sullivan
Cc: "Rickard Sjöberg (KI/EAB)"; mp4-tech lists.mpegif.org; Tung yi-Shin;
Yi-Shin Tung; ohm ient.rwth-aachen.de
Subject: Re: [Mp4-tech] Regarding padding in MPEG-4 part 2
Gary, Yi-Shin, Jens,
Please look at the 3rd (2004) edition:
The sentence
"Note that for rectangular VOP, a reference VOP is defined by
video_object_layer_width and video_object_layer_height."
in the 2nd edition was changed to
"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"
in the 3rd edition.
I guess this schould make you happy, Gary :-)
Regards,
Herbert.
Gary Sullivan wrote:
> Copying Yi-Shin and Jens, who perhaps remember this topic,
>
> Actually the action I discussed may have happened longer ago than that. I
looked in N6362
> and although it touched on some relevant sections (I think including
7.6.3) I did not quite
> find an answer to the question there. I also looked in N3664 and didn't
find it there either.
> I then looked in the text of the 2nd (2001) edition and although I got a
bit confused by what it
> says in various plances (although I haven't really studied the topic fully
yet), I think I found it.
>
> What I think I see supports what I said on this thread. At the end of
7.6.4 in the 2nd edition
> it says the following:
>
> xref = MIN ( MAX (xcurr+dx, vhmcsr), xdim+vhmcsr-1 )
> yref = MIN ( MAX (ycurr+dy, vvmcsr), ydim+vvmcsr-1 )
>
> and
>
> "(ydim, xdim) are the dimensions of the bounding rectangle of the
reference VOP"
>
> and
>
> "Note that for rectangular VOP, a reference VOP is defined by
video_object_layer_width and video_object_layer_height."
>
>
> Now we know that video_object_layer_width and video_object_layer_height
might not be multiples of 16.
>
> Based on those equations and quoted sentences, it sounds to me like
padding is applied for any location beyond the rectangle having (width,
height) = (video_object_layer_width, video_object_layer_height).
>
> This is not something that I am happy about, but I think it is what is
currently written and I think there was some historical corrigendum action
that changed it to say that.
>
> There has been a long history of confusion over this issue. I am not
entirely sure that I have read all relevant parts of the spec, but I don't
see how the above quoted statements can be interpreted any other way.
>
> Best Regards,
>
> Gary Sullivan
>
>
> +> -----Original Message-----
> +> From: Gary Sullivan
> +> Sent: Thursday, November 23, 2023 2:20 AM
> +> To: 'Herbert Thoma'
> +> Cc: "Rickard Sjöberg (KI/EAB)"; mp4-tech lists.mpegif.org
> +> Subject: RE: [Mp4-tech] Regarding padding in MPEG-4 part 2
> +>
> +> 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
> +> +> > +>
> +> +> > +> _______________________________________________
> +> +> > +> 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
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> +> +> > +>
> +> +> >
> +> +> > _______________________________________________
> +> +> > 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/
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