W3C home > Mailing lists > Public > whatwg@whatwg.org > November 2008

[whatwg] video tag: pixel aspect ratio

From: Pierre-Olivier Latour <pol@apple.com>
Date: Mon, 17 Nov 2008 13:58:42 -0800
Message-ID: <9D4E2546-8ADF-48B5-BCFC-BDF0E1FC6960@apple.com>
>> 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.

________________________________
Pierre-Olivier Latour - pol at apple.com
Rich Media Team - Apple, Inc.


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