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

[whatwg] video tag: pixel aspect ratio

From: Pierre-Olivier Latour <pol@apple.com>
Date: Thu, 13 Nov 2008 21:12:57 -0800
Message-ID: <BB1F7605-C03E-4C6C-992A-09309B34FFA9@apple.com>
>> I agree that incorrectly encoded videos are annoying, but I don't  
>> think
>> we should have this attribute at all because I don't think it  
>> passes the
>> "will it be commonly used" smell test.
>>
>> I am also afraid that it will difficult to use correctly, since you
>> frequently have to use clean aperture in conjunction with pixel  
>> aspect
>> ratio to get the correct display size.
>
> For the videos we're talking about, just getting near the right  
> ratio is
> probably all we can ask for -- we're not talking about professional  
> video
> data here. We're talking misencoded YouTube videos where an embedder  
> wants
> to fix the most egregious error before showing his friends the cat  
> jumping
> off the side of the pool or something.
>
> I agree that this is just a hack attribute, and I agree that it isn't
> going to be widely used. But I think it will be used enough to  
> justify its
> existence. There are a surprisingly large number of misencoded  
> videos on
> the Web, and plenty of people who care.


I don't see how people who can't properly transcode (the majority of  
users I guess), will know on the other hand which aspect ratio to use  
to fix the problem (or even think about using this fix). Maybe  
there'll be some JS libraries around to do this automatically on the  
fly, but I don't see how this would ever be consistent or reliable.  
The real fix would need to be done in their workflow or tool they use.  
Also wouldn't services like YouTube be able to auto-detect such videos  
and resize them anyway (uploaded QT movies should have all the  
necessary info embedded for instance, but even if it is missing, you  
can likely assume that a 720x480 video should be resized to 640x480  
before being served)?

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.
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...

At the same time, saying that pixel ratio is intentionally ill-defined  
because we don't *really* want people to use it is also quite confusing.

If we start going in this direction, then <img> should have a "dpi"  
attribute so you can "hack" around images uploaded at dpi > 72 ;)

In any case, if this attribute really needs to be present, we should  
rename it at the minimum (picking a term from the "professional" video  
world requires taking the constraints that come with it), maybe  
"displayRatio" or something?

________________________________
Pierre-Olivier Latour - pol at apple.com
Rich Media Team - Apple, Inc.
Received on Thursday, 13 November 2008 21:12:57 UTC

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