W3C home > Mailing lists > Public > public-html@w3.org > August 2013

Re: Dropping cues when playbackRate != 1.0

From: Eric Carlson <eric.carlson@apple.com>
Date: Fri, 16 Aug 2013 10:17:53 -0700
Cc: Brendan Long <self@brendanlong.com>, "HTML WG (public-html@w3.org)" <public-html@w3.org>
Message-id: <4DF6785C-EAEA-4644-AA4E-5721A8CB44D4@apple.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>

On Aug 8, 2013, at 7:21 PM, Silvia Pfeiffer <silviapfeiffer1@gmail.com> wrote:

> On Fri, Aug 9, 2013 at 6:11 AM, Brendan Long <self@brendanlong.com> wrote:
>> There's section of the HTML5 CR spec saying:
>> 
>> Similarly, when the playback rate is not exactly 1.0, hardware, software, or
>> format limitations can cause video frames to be dropped and audio to be
>> choppy or muted.
>> 
>> 
>> I think this should also apply to text track cues, something like:
>> 
>> Similarly, when the playback rate is not exactly 1.0, hardware, software, or
>> format limitations can cause video frames and text cues to be dropped and
>> audio to be choppy or muted.
>> 
>> When playing at non-standard speeds, an efficient implementation may want to
>> skip portions of the file, which could mean skipping cues.
> 
> I assume you are talking about a situation where the playback would
> need to jump multiple seconds (call them "s") at a time to achieve the
> speedup.
> And you are further assuming that when the current time jumps from
> second x to second x+s, there may be a cue that would have both their
> start and end time between x and x+s.
> So you're saying that if there is no audio or video rendered to which
> the cue refers, the "time marches on" algorithm doesn't actually
> activate these cues and therefore they are "dropped".
> 
> In actual fact, the "time marches on" algorithm will activate the
> enter and exit events of the "missed cues", so they are not really
> "dropped". They are, however, not rendered, because they don't become
> active.
> 
  This requires that all of the media data for time X to X+S has to be loaded before skipping ahead. This means that a UA can not play at a faster rate by not loading the entire file, for example by only loading and displaying key frames. 

  If this is allowed, and it seems silly to prevent it, cues defined in the portions of the file that are not loaded would have to be skipped.

eric
Received on Friday, 16 August 2013 17:18:22 UTC

This archive was generated by hypermail 2.3.1 : Thursday, 29 October 2015 10:16:34 UTC