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Re: Change Proposal for ISSUE-147

From: Philip Jägenstedt <philipj@opera.com>
Date: Thu, 17 Mar 2011 20:46:09 +0100
To: public-html@w3.org
Message-ID: <op.vsh6i70rsr6mfa@nog>
On Sat, 05 Feb 2011 04:23:13 +0100, Frank Olivier  
<Frank.Olivier@microsoft.com> wrote:

> Change Proposal for ISSUE-147
> playbackrate: undefined behavior when the user agent can't play back at  
> the requested rate
>
> Summary
> The specification should describe what to do if the user agent is unable  
> to play media at the requested rate. This change proposal requests that  
> the spec be changed so that if the user agent cannot support the  
> requested playbackRate then this should be ignored and the value of  
> playbackRate remain unchanged. See  
> http://dev.w3.org/html5/spec/video.html#dom-media-playbackrate.
>
> Rationale
> Implementations in browsers vary when an unsupported playback rate is  
> requested. Since different implementations will have different  
> capabilities it is important that the media element responds in a  
> reliable way so that script can detect the issue.
> There are a number of reasons why a user agent may not be able to play a  
> resource at the requested speed. For example:
> . On low power devices playback is sometimes off-loaded to hardware that  
> is optimised for forward playback that does not efficiently support  
> reverse playback.
> . Some media formats are not designed to be efficiently played in  
> reverse.
> . Setting playbackRate larger than 1.0 for live video will not work.
>
> Details
> In section  
> http://dev.w3.org/html5/spec/video.html#dom-media-playbackrate add the  
> italicized text.
> The attribute is mutable: on getting it must return the last value it  
> was set to, or 1.0 if it hasn't yet been set; on setting the attribute  
> must be set to the new value, and the playback must change speed (if the  
> element is potentially playing). If the user agent is unable to play at  
> the requested speed the set should be ignored and the attribute remain  
> at the previous value.
> Impact                                                                                                                                      
> Positive Effects
> . Script can reliably detect when an unsupported playbackRate has been  
> requested.
> Negative Effects
> . None.
>
> Conformance Classes Changes
> Web browsers and other interactive user agents.
> Risks
> None. Browsers already vary in this respect.
> References
> . http://dev.w3.org/html5/spec/video.html#dom-media-playbackrate
>

This CP assumes that the UA knows beforehand which playback rates it can  
support. Like many things in media, the only way of knowing for sure may  
be to try it, so how should a UA handle a situation like that?

-- 
Philip Jägenstedt
Core Developer
Opera Software
Received on Thursday, 17 March 2011 19:46:47 GMT

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