[whatwg] video tag: pixel aspect ratio

Lachlan Hunt lachlan.hunt at lachy.id.au
Mon Dec 1 14:07:19 PST 2008

Philip Jägenstedt wrote:
> The use case for stretching moving images (<video>) is exactly the 
> same as for stretching animations (<img src=animation.gif>) or static 
> images (<img src=static.jpg>).

Consistency is not a use case.  For images, we're constrained by 
backwards compatibility requirements to stretch them, rather than keep 
aspect ratios, regardless of what use cases there may or may not be for 
doing so.  For video, we are not constrained by such backwards 
compatibility and we have the opportunity to specify the most rational 
alternative as the default.

> While I think that fixing incorrect aspect ratio is the most probable 
> use, consistency and generality is the real issue here.

Given that the pixelratio attribute was dropped in part due to a lack of 
compelling use cases, I don't think the same use case will be compelling 
for this either.

If there are real use cases for stretching video, then they should be 
judged on their own merits, rather than just falling back to the 
consistency argument.

> With an image of size 800x600 it's possible to display at any size, 
> even those which don't match the aspect 4:3. With moving images 
> (video) we can't influence it at all.
> The previous solution was the pixelratio attribute, but I have 2 
> other possible solutions:
> <video src="4x3.mpg" width="1920" height="1080" keepaspect>
> The video would be pillarboxed.

Retaining the aspect ratio needs to be the default because based on 
experience with video on the web today, e.g. on YouTube, and also based 
on the behaviour of common media players, like VLC, QuickTime, Windows 
Media, etc.  When a user resizes their video player, the video generally 
maintains the aspect ratio and gets black bars; (or in QuickTime's case, 
the window size is constrained by the video's aspect ratio).  There are 
menu options in VLC, and possibly others, to set the aspect ratio of a 
video to a selection of common aspect ratios, but there is no arbitrary 
scaling (at least, not that I'm aware of).

> <video src="4x3.mpg" width="1920" height="1080">
> The video would be stretched and have incorrect aspect

And if a user chooses to put the video in full screen, then what? Should 
it maintian that incorrect aspect ratio?  Should it stretch to fill the 
whole screen, regardless of what type of monitor the user is using, or 
should it use the video's default aspect ratio?

> Given the number of people who watch 4:3 video stretched to a 16:9 
> display without even noticing/caring that the aspect ratio is wrong,

While I'm aware that there are such people, largely because they don't 
know how to configure their TV's properly or because their TV's are 
broken by design, there are many others who do notice the difference 
(including myself).  From my experience, stretching a 4:3 picture to 
fill a 16:9 screen is enough to make people on the screen look weird and 
out of proportion.  It's less noticeable with cartoons, but in general, 
it's very noticeable and annoying.  We should inflict such annoyances 
upon end users if it avoidable; at least not by default.

Lachlan Hunt - Opera Software

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