[whatwg] [media] startOffsetTime, also add startTime?

Philip Jägenstedt philipj at opera.com
Thu Apr 5 02:07:17 PDT 2012

On Wed, 04 Apr 2012 20:56:03 +0200, Ian Hickson <ian at hixie.ch> wrote:

> On Wed, 4 Apr 2012, Philip Jägenstedt wrote:
>> > >
>> > > In current Opera and Firefox the timeline is always normalized to
>> > > start at 0, so the time that corresponds to 0 in the original
>> > > timeline would be at a negative currentTime.
>> >
>> > I still don't really understand what you mean by "start" here.
>> What I mean with "normalized to start at 0" is that when playing the
>> whole resource
> Now I don't understand what you mean by "whole resource". :-)
> If a TV station starts broadcasting in October, and you join the stream  
> in
> February, the "whole resource" is many months long. Where's your zero?
> (I guess it's academic if you plan on changing; I'm just curious.)

Oh, "whole resource" is a bit of a misnomer, I really mean "everything  
that we received". We don't make that distinction internally, since over  
HTTP there's no way to tell the difference between a streaming resource  
and a static resource served by a server that doesn't support byte ranges.  
The timeline is shifted by the timestamp of the first audio or video  
sample in the stream, such that currentTime is initially 0. The plan is to  
remove that adjustment.

>> The spec changes around explicit timelines and static/streaming
>> resources are a big improvement, thanks! However, it now talks about
>> both "explicit timeline" and "explicit timings" in a way that makes me
>> uncertain about Ogg. Ogg (at least without skeleton) is just a stream of
>> timestamped packets, so the timeline simply spans the timestamp of the
>> first packet to the timestamp of the last packet. WebM is similar in the
>> streaming case in that timestamps the don't start at 0. Clarification of
>> whether or not "explicit timestamps" (Ogg, WebM) implies an "explicit
>> timeline" would be welcome. I assume that's the intention, which I also
>> agree with. (Perhaps saying "explicit frame durations" instead of
>> "explicit timings" would also help.)
> I've tried to change this.

Excellent! For anyone still reading, the new text is:

"If the media resource somehow specifies an explicit timeline whose origin  
is not negative (i.e. gives each frame a specific time offset and gives  
the first frame a zero or positive offset), then the media timeline should  
be that timeline."

Philip Jägenstedt
Core Developer
Opera Software

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