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Re: Buffered bytes for media elements

From: Eric Carlson <eric.carlson@apple.com>
Date: Thu, 16 Oct 2008 19:17:01 -0700
To: John Harding <jharding@google.com>
Cc: Ian Hickson <ian@hixie.ch>, Dave Singer <singer@apple.com>, public-html@w3.org
Message-id: <1B90CE7C-545B-4D63-B11C-59834F26D81B@apple.com>

On Oct 15, 2008, at 11:44 AM, John Harding wrote:

> Some background behind the request:
> It is common for web sites displaying video (such as YouTube) to  
> control the buffering and playback of video in order to optimize the  
> user experience.  One example is deciding when sufficient data has  
> been buffered to begin playback - while the spec currently include  
> the "canPlayThrough" event, this isn't really flexible enough for  
> all use cases.  Users often do not watch entire videos, so delaying  
> playback until complete playthrough is possible is too much of a  
> delay.  It's often appropriate to play some video as soon as  
> possible, and modify buffering behavior during playback as the  
> user's intentions become more clear.
> Another example is that web sites may have multiple versions of  
> videos, and want to be able to make their own determination of when  
> to switch from one to another, vs. leaving that up to the user agent.
>
> While the buffered time ranges provide an approximation to this,  
> they can be very far off the mark for some types of content.   
> Similarly, there are scenarios where buffered byte counts are also  
> incorrect, but the overwhelming majority of the time, video will be  
> a static file served by a standard server, such that it's trivial  
> for the agent to provide that data.
>
   I don't follow your logic. How can knowing the number of *bytes*  
available allow you to make a good decision about when sufficient data  
is available to begin playback when you don't know the bit-rate of the  
media file? In other words, how can you know when it is safe to begin  
playback when you have N bytes available but you don't know if playing  
the media file requires 10 K per second or 10 Megs per second?

   I would think that just monitoring the number of seconds of media  
available as time passes would give you a *more* accurate picture  
about when it makes sense to ask the server for a lower bit-rate  
version - "oh oh - this user is only buffering 1 second of media every  
5 seconds, time to switch to the smaller version".

   I may be missing something obvious, but I can't see how having  
information about the number bytes of a time-based resource is useful.
Can you spell out the use case(s) for these properties in more detail?

   Thanks,

eric

>
>
> On Mon, Oct 13, 2008 at 6:06 PM, Ian Hickson <ian@hixie.ch> wrote:
> On Thu, 18 Sep 2008, Dave Singer wrote:
> >
> > <http://www.w3.org/html/wg/html5/#media>
> >
> > From the spec.:
> >
> # The bufferedBytes attribute must return a static normalized   
> object that
> # represents the ranges of the media resource, if any, that the user  
> agent
> # has buffered, at the time the attribute is evaluated.
> #
> # The totalBytes attribute must return the length of the media  
> resource,
> # in bytes, if it is known and finite. If it is not known, is infinite
> # (e.g. streaming radio), or if no media data is available, the  
> attribute
> # must return 0.
> >
> > We don't think these are well-defined for a whole host of cases:
> > -- live streams
> > -- SMIL files that reference several media files
> > -- media files like MOV and MP4 that reference media files, or  
> even MP4 or MOV
> > files that are not interleaved in time order
> > -- streaming protocols in general (non-buffering)
> >
> > It's by no means clear to us what these attributes are for -- what  
> the use
> > cases are.  We think they should be removed, or supported with use  
> cases that
> > are able to show why they are useful despite all these cases where  
> either
> > their meaning or utility (or both) are unclear...
>
> These attributes were added primarily in response to a request from
> YouTube, if I recall correctly. The problem they solve is that there  
> can
> be a great difference between the amount of time buffered and the  
> number
> of bytes buffered, and a clever author would find information about  
> the
> bytes buffered to be far more useful than the amount of time  
> available.
>
> I agree that for many use cases, these are underdefined. I would be
> interesting in hearing feedback on what would be the best way to  
> resolve
> this problem. If we remove the feature altogether, we should have an
> alternative of some kind, though.
>
> --
> Ian Hickson               U+1047E                ) 
> \._.,--....,'``.    fL
> http://ln.hixie.ch/       U+263A                /,   _.. \   _ 
> \  ;`._ ,.
> Things that are impossible just take longer.   `._.-(,_..'-- 
> (,_..'`-.;.'
>
Received on Friday, 17 October 2008 02:17:58 UTC

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