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Should <video> buffer control be tri-state?

From: Maciej Stachowiak <mjs@apple.com>
Date: Sat, 02 Jan 2010 18:42:26 -0800
Cc: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, Philip Jägenstedt <philipj@opera.com>, Edward O'Connor <hober0@gmail.com>, Jeremy Keith <jeremy@adactio.com>, HTMLwg <public-html@w3.org>
Message-id: <E6BE5341-C898-4AB1-A9EE-89D12469AE0C@apple.com>
To: "Tab Atkins Jr." <jackalmage@gmail.com>

I'm picking a semi-arbitrary point to fork a new thread.

On Dec 30, 2009, at 9:17 AM, Tab Atkins Jr. wrote:

> On Wed, Dec 30, 2009 at 7:33 AM, Silvia Pfeiffer
> <silviapfeiffer1@gmail.com> wrote:
>> 3. @autobuffer="off" - the web page author cares and believes it  
>> would
>> be a nuisance to the user to autobuffer this video and waste
>> bandwidth, so the browser doesn't autobuffer - it this case, it may
>> even make sense to not even try and initialise the decoding pipeline,
>> but only display the poster frame, if possible (maybe the
>> X-Content-Duration can help to display the video duration and that's
>> all that's required?). I'm specifically thinking here about a Web  
>> page
>> that has dozens of videos on it (e.g. as search results or for
>> browsing a collection). It might not make sense to pre-buffer  
>> anything
>> at all in such a case where playback is highly unlikely.
>
> This comes back around to what Philip was saying before - is it really
> best for the user to treat autobuffer=off any different than no
> autobuffer at all?  *Will* a browser treat the two differently?  We
> know that it's obviously often best to not autobuffer by default -
> this has been established, and the automatic autobuffering that is
> going on currently in some browsers is a bug.  Once that is fixed,
> though, is it really worthwhile to do something fully different in
> your situation 1 and 3?

Buffering by default isn't a clear win or lose - it's a tradeoff. When  
a page has only a single video that the user is highly likely to want  
to play, and the user has a good network connection, it's probably  
best to buffer aggressively, so the user gets a faster response when  
they choose to play. If a page has many <video> elements, then it's  
probably best not to buffer aggressively, as the "thundering herd" of  
multiple videos being buffered will likely swamp their system, and the  
likelihood of buffering any one would be slow.

In the absence of any explicit attributes for buffer control, a likely  
good design would be to apply a heuristic. For example: if a page  
contains only one <video> element, then buffer aggressively. If it  
contains many, don't buffer any of them. Alternately, one could look  
at whether a particular video has larger explicit dimensions or  
appears in a more prominent place on the page. Since an unaware author  
is most likely not to add any special attributes, it would be nice to  
apply a heuristic like this when no special buffering-related  
attribute is present. Let's call this case (A).

This leaves two other cases: (B) author knows a video is very likely  
to be played, notwithstanding what browser heuristics may guess; in  
this case he or she adds the "autobuffer" attribute; (C) author knows  
a video is not that likely to be played, even though it may otherwise  
appear so to heuristics -- for example, a blog may contain a video in  
only one posting but the blog author thinks most readers will not play  
it on the typical visit. In case (B) the browser should do its best to  
buffer, given bandwidth constraints, and in case (C) it should almost  
certainly not buffer.

But right now there is no way to distinguish case (C) from case (A).  
So when autobuffer is omitted, browsers are faced with the choice of  
either never buffering, even if the omission is solely through lack of  
awareness, or applying heuristics even when the page author explicitly  
does not want buffering.

Thus, I think it would be best to add a nobuffer attribute or  
autobuffer=off or something like that to give a strong hint that a  
video's full data will not be needed, in the same way that autobuffer  
hints that the video's full data likely will be needed.

Regards,
Maciej
Received on Sunday, 3 January 2010 02:43:00 UTC

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