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[whatwg] video tag: pixel aspect ratio

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Tue, 18 Nov 2008 12:37:45 +1100
Message-ID: <2c0e02830811171737n41acd35fl981434ef59138acd@mail.gmail.com>
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?

Regards,
Silvia.
Received on Monday, 17 November 2008 17:37:45 UTC

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