W3C home > Mailing lists > Public > public-html-a11y@w3.org > April 2011

Re: [media] change proposals for issue-152

From: Eric Carlson <eric.carlson@apple.com>
Date: Thu, 31 Mar 2011 19:58:27 -0700
Cc: Ian Hickson <ian@hixie.ch>, Mark Watson <watsonm@netflix.com>, "public-html-a11y@w3.org" <public-html-a11y@w3.org>
Message-id: <BCECD1A7-EA5A-4E46-A85C-2ABA89DCF769@apple.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>

On Mar 31, 2011, at 3:41 PM, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:

> On Fri, Apr 1, 2011 at 1:49 AM, Eric Carlson <eric.carlson@apple.com> wrote:
>> 
>> On Mar 30, 2011, at 9:22 PM, Silvia Pfeiffer wrote:
>> 
>>> But I am told, UAs render them simply over each other
>>> at the top left corner. In any case, there will be a default rendering
>>> IIUC.
>>> 
>> This is not quite right. Every visual track in MPEG-4 and QuickTime containers, at least, has a display matrix that determines where it is rendered. The movie's display box is the union of all visual track display boxes, eg. the size and position of a visual track affects the size of its movie.
>> 
>> It is certainly *possible* to render an in-band track in the top left corner, and that may be the default in some media authoring software, but it is not a requirement.
> 
> 
> Thanks for the clarification. It confirms though that multiple in-band
> video tracks are indeed rendered by default into the existing video
> viewport unless they are somehow turned off.
> 
Correct. 

> I now wonder: is there actually a means to turn them off and just use
> them in a separate audio or video element with a fragment identifier?

Using an audio track in another element is easy as long as it will be played in sync with the other elements(s) that use the same file. If it could possibly be played out of sync, it will be necessary to open up another instance of the file and enable/disable tracks appropriately. 

Some media engines allow a client to render each track to a separate bitmap/surface, so what I wrote about audio can also be true for video tracks. However, some media engines always composite video tracks, so it a client of one of these will always have to open up a separate copy of the file for each video track they want to render. 

> In multitrack in-band resources, are the multiple media tracks
> typically all activated by default or how is the decision made whether
> to render them?
> 
A track's metadata specifies its initial state (enabled, display matrix, volume, etc), so the file's author is in control. 

eric
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Received on Friday, 1 April 2011 03:30:14 UTC

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