[whatwg] video tag: pixel aspect ratio

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Mon Nov 17 17:37:45 PST 2008

On Tue, Nov 18, 2008 at 8:58 AM, Pierre-Olivier Latour <pol at apple.com> wrote:
> And the suggested "hack" is not even really usable: if you have a video
> coming
> from a NTSC DV source as 720x480 improperly transcoded to say MP4 720x480
> square pixels, using the theoretical 10:11 pixel aspect ratio will _not_
> make
> it look right: it needs to be clipped to 704x480 first.
> Are you sure? If you don't clip it, you still get the right shape pixels,
> don't you? You don't get the right final video size, sure, because you
> didn't crop, but so what? We're just trying to do a last-ditch aspect
> ratio fix here, not get perfect video.
> Well, the pixels will look right if you pass 10:11, but not the overall
> video, or the video will look right but not the pixels if you pass an aspect
> ratio to end up with 640x480 (the very nice 0.888888888888889)...
> Pixel aspect ratio has a precise meaning in the video world, and using
> it outside of clean aperture does not make a lot of sense...
> As far as I can tell, using it outside clean aperture works fine so long
> as you don't also expect the final output to be the "right" video size.
> You're effectively saying that it works *fine* as long as you we don't
> expect to work *right*. I have to admit, this is a concept that escapes me
> ;)
> If we start going in this direction, then <img> should have a "dpi"
> attribute so you can "hack" around images uploaded at dpi > 72 ;)
> We effectively do, it's the "height" (or "width") attribute.
> Exactly my point: now replace, <img> by <video>, "dpi" by "aspectRatio" and
> add a new boolean attribute to the video tag, so you can do "fillToFit"
> instead of "scaleToFit" and you have a real solution that allows you to
> resize the video the way you want and avoids half-baked concepts like "it's
> pixel aspect ratio, but actually not really, and you shouldn't be using it
> anyway".
> Personally I don't really see the problem with "pixelratio".
> I might be missing something here, but:
> 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 was under the impression that the attribute was requested by
YouTube. Does YouTube itself provide such an attribute? If now, why
not? If so, how often is it being used?


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