[M4IF Technotes] MPEG4 Visual: Aspect Ratio

[Gary Sullivan]

Rob's answer is completely correct.  Decoders must decode
weird bitstreams that conform, but what to do for display
is entirely up to the decision of the decoding system.
-Gary
+> -----Original Message-----
+> From: Rob Koenen [mailto:rkoenen   intertrust.com] 
+> Sent: Tuesday, March 05, 2024 2:14 PM
+> To: 'Christoph.Stadler   dynapel.de'; technotes   lists.m4if.org
+> Subject: RE: [M4IF Technotes] MPEG4 Visual: Aspect Ratio
+> 
+> 
+> I never ssaw an answer to this question.
+> You are right in stating that conformant decoders MUST be able
+> to decode these 'other' aspect ratios. Do the decoders out there
+> indeed decode them? that's up to the decoder manufacturers to
+> answer.
+> 
+> The standard, however, prescribes nothing about display. So 
+> the decoded picture may not fit on your display, or you may
+> give the user the option to pan. On a computer display, you can
+> actually do pretty neat things with 'other' display ratios
+> (and with the option to have multiple objects for that matter)
+> 
+> 
+> Rob
+> 
+> > -----Original Message-----
+> > From: Christoph Stadler [mailto:Christoph.Stadler   dynapel.de]
+> > Sent: Friday, February 22, 2024 9:13
+> > To: technotes   lists.m4if.org
+> > Subject: [M4IF Technotes] MPEG4 Visual: Aspect Ratio
+> > 
+> > 
+> > 
+> > Dear M4IF community,
+> > 
+> > I have a question concerning the pixel aspect ratios used in
+> > MPEG-4 Visual:
+> > 
+> > The bitstream syntax allows to either use one of the pre-defined
+> > pixel aspect ratios (e.g. 12:11 or 40:33) or to explicitly specify 
+> > a private one by setting 'aspect_ratio_info:=extended_PAR'. 
+> > This allows to transmit any aspect ratio that can be represented 
+> > as a fraction p:q with p,q in [1;255].
+> > 
+> > Although this sounds to me like a nice and possibly very helpful 
+> > feature, I somewhat hesitate to use it in a real encoder, as I fear
+> > that the (unspecified) decoder - or its attached 
+> compositor/display 
+> > unit - will not be able to handle these arbitrary values. 
+> > 
+> > Of course this would imply that the decoder/compositor is 
+> not fully 
+> > conformant, but if it works properly with all the other 
+> encoders which
+> > use only the standard aspect ratios, my position could turn 
+> > out rather 
+> > weak.
+> > 
+> > Does anyone of you have experiences confirming or dispersing my 
+> > worries ?
+> > 
+> > Thank you in advance,
+> > 
+> > 	Christoph Stadler
+> > 
+> > 
+> > ---------
+> > DynaPel Labs GmbH                Tel. +49 (0)89 962428-25
+> > Dr. Christoph Stadler
+> > Fraunhofer Strasse 9             eMail christoph.stadler   dynapel.de
+> > D-85737 Ismaning
+> > GERMANY                          www.dynapel.de
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