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Re: Dropping cues when playbackRate != 1.0

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Thu, 22 Aug 2013 23:32:29 +1000
Message-ID: <CAHp8n2kNwxtJHaMfx8C_Dx6dw9C5SiMtfBTjOg_q7XeUotphpA@mail.gmail.com>
To: Eric Carlson <eric.carlson@apple.com>
Cc: Brendan Long <self@brendanlong.com>, "HTML WG (public-html@w3.org)" <public-html@w3.org>
On Thu, Aug 22, 2013 at 10:28 AM, Eric Carlson <eric.carlson@apple.com>wrote:

>
> On Aug 21, 2013, at 5:12 PM, Silvia Pfeiffer <silviapfeiffer1@gmail.com>
> wrote:
>
>
>
>
> On Sun, Aug 18, 2013 at 12:42 AM, Eric Carlson <eric.carlson@apple.com>wrote:
>
>>
>>
>> On Aug 17, 2013, at 5:40 AM, Silvia Pfeiffer <silviapfeiffer1@gmail.com>
>> wrote:
>>
>>
>>
>>
>> On Sat, Aug 17, 2013 at 3:17 AM, Eric Carlson <eric.carlson@apple.com>wrote:
>>
>>>
>>> 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.
>>
>>
>> Why? Only the media data for time x and for time x+s has to be loaded -
>> nothing else.
>>
>>
>>
>>> 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.
>>>
>>
>> I guess we have two different kind of cues: those coming from <track> and
>> those coming from inside a media file.
>>
>> Those from a <track> element are all loaded and events could be activated
>> as the timeline skips over the cue start/end times. Cue content would be
>> rendered when X or X+S is within a cue's interval.
>>
>> Those from inside a media file, if multiplexed, may get completely
>> skipped by going from X to X+S without the browser loading those parts of a
>> file. So, in this case you'd probably skip cues.
>>
>>   Yes, sorry for being unclear, I was talking about in-band captions.
>>
>>
> I'm trying to figure out whether this may be a problem, both because it's
> inconsistent between in-band and <track> based text tracks, and because JS
> developers may need to be notified of skipped cues.
>
> In your experience, for in-band tracks, would it be possible to require
> that the browser not fast-forward text tracks, but only skip audio / video
> track sections?
>
>   Not unless we disallow seeking to an un-buffered time. There is no
> chance I would make a change like that.
>
> I guess it would require writing some sort of index of the cues into the
> file. It's possible in Ogg with a special Skeleton header for text tracks.
>
>   That won't help because the index will only give you the cue timestamps,
> not the text.
>

That would be enough to raise events for skipped cues.

Say... if you are skipping to a video segment and there is a caption cue
that is active during the time of the video segment, will you get it or
miss it? Are you doing the fast forward/reverse by skipping byte ranges or
by skipping via an index?

If you have an index of the locations in the file at which the cues are,
both as a time and byte offset, then you can make sure not to seek past any
cues. So, I think it could work. It would, of course, all depend on how the
fast forward/reverse is implemented.

If we can raise enter/exit events for cues while seeking, I think we can
assume that to mean that the cues aren't skipped.

Right now, I think Safari is the only browser that supports fast
forward/reverse for video. If your index on in-band text tracks can't
support accessing each cue on seek, or at minimum raising events for
skipped cues, then I think we do need to make allowances in the text.

Silvia.
Received on Thursday, 22 August 2013 13:33:21 UTC

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