[M4IF Technotes] MPEG4 Visual: Aspect Ratio

[Christoph Stadler]

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