[whatwg] video tag: pixel aspect ratio

Philip Jägenstedt philipj at opera.com
Mon Nov 17 14:32:19 PST 2008

I should point out that the pixelratio attribute isn't only for authors,
it's also useful when the media framework used doesn't recognize the
(pixel) aspect ratio even when it's correctly set. From reading the
mplayer man page I see that AVI files can have aspect ratio set in the
OpenDML vprp header, but I doubt e.g. DirectShow supports this (I could
be wrong though).

I don't feel very strongly about the attribute either way, but given
that video is scaled to fit inside its element box with aspect preserved
and not simply stretched then an incorrectly parsed/guessed aspect ratio
would make a big difference. This seems very similar to the width/height
attributes of an image and that usually isn't considered an ugly hack.
If the pixelratio attribute is removed then I would suggest also
removing the involvement of aspect ratio over the size of the element.

By the way, the "pixel-aspect-ratio" on video caps in the GStreamer
framework has precisely the same meaning as this attribute, overriding
it on a video sink also has an effect similar to what is suggested in
the HTML5 spec. In other words, it's not so outlanding from a media
framework's point of view.

Oh, and if the video also needs cropping to get the correct aperture,
couldn't this be done with CSS perhaps?


On Mon, 2008-11-17 at 14:02 -0800, Peter Kasting wrote:
> On Mon, Nov 17, 2008 at 1:58 PM, Pierre-Olivier Latour <pol at apple.com>
> wrote:
>         1) I don't remember any major media system I've dealt with so
>         far having an explicit pixel aspect ratio override API,
>         2) on the web, neither QT plug-in nor Flash have it,
>         3) in the case of this spec, the way it's defined makes it
>         behave incorrectly
>         4) it's not straightforward to use (see very first reply
>         above)
>         5) there's no _actual_ data that proves it's necessary
>         (shouldn't the software or video web site fix the videos
>         upfront?)
>         Based on this, it seems to me this attribute should not be in
>         the spec by default, and we should switch the burden of the
>         proof to people who want it (rather than it being on people
>         who don't want it as it seems to be the case today), and
>         finally wait to see 1) if there's a real need for a solution
>         here and 2) if the best solution is indeed a pixel aspect
>         ratio override.
> I agree.  The more this attribute is discussed, the more "this is a
> hack that no one should actually use" starts to sound like "we
> shouldn't put this in the spec to begin with".  The potential for
> problems seems greater than the upside from authors correctly using
> this to do emergency-overrides of particular videos whose sources they
> don't control.
> PK 
Philip Jägenstedt
Opera Software

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