Re: UAAG 2.0 guidelines for video

On Wed, 05 May 2010 07:23:45 +0800, Silvia Pfeiffer  
<silviapfeiffer1@gmail.com> wrote:

> Hi Jim,
>
> I have read of all these requirements. It's quite an impressive list
> and will certainly be useful to John as he is putting together the
> requirements list discussed in the last phone conference of the media
> a11y subgroup in the HTML5 a11y TF.
>
> I believe that most of these requirements are already being taken care
> of or are in development - much of it is not yet present in browsers,
> but that is more an implementation issue than a specification issue.
> It is probably important for this group to continue to monitor
> progress on these issues and to remind browser vendors to implement
> these required features.
>
> I am, however, concerned about the  following two requirements, since
> they are fundamental requirements on media resources that are not
> regarded in today's HTML5 media elements and I am not aware of any
> plans to introduce them.
>
>
> On Wed, May 5, 2010 at 4:50 AM, Jim Allan <jimallan@tsbvi.edu> wrote:
>>
>> 4.9.5 Playback Rate Adjustment for Prerecorded Content: The user can  
>> adjust
>> the playback rate of prerecorded time-based media content, such that  
>> all of
>> the following are true (Level A):
>>
>>    * The user can adjust the playback rate of the time-based media  
>> tracks
>> to between 50% and 250% of real time.

The spec already requires browsers to handle *all* rates, from -Infinity  
to +Infinity.

>>    * Speech whose playback rate has been adjusted by the user maintains
>> pitch in order to limit degradation of the speech quality.

Unless the browser has some way of knowing that the audio is speech, the  
only thing one can do is apply pitch correction for all audio. I know some  
version of WebKit does something like this, and would support a DOM  
attribute to give authors explicit control over it.

>>    * Audio and video tracks remain synchronized across this required  
>> range
>> of playback rates.

The spec already requires audio/video sync to be maintained for *all*  
rates.

>>    * The user agent provides a function that resets the playback rate to
>> normal (100%).

Sure, that's doable, but might clutter up UI/context menus.

>> 4.9.11 Adjust Playback Contrast and Brightness. User has ability to  
>> control
>> the contrast and brightness of the content within the playback viewport.
>
> This is actually a very similar issue to the playback rate. Again, we
> have to answer the question whether this is something that we want to
> give the Web page author the ability to influence or just something
> between the UA and the user.

This is a bit tricky. Since the <video> may be drawn to a <canvas> at any  
time, the browser would either have to keep track of two video frames (one  
unadjusted and one adjusted) or make use of 3D hardware that can apply the  
adjustment very late in the rendering as a fragment shader or similar. On  
some devices neither of these will be possible, so this should at most be  
a "nice to have" feature.

-- 
Philip Jägenstedt
Core Developer
Opera Software

Received on Wednesday, 5 May 2010 03:48:36 UTC