Re: Do we need to support media that doesn't start at 0?

On Wed, Apr 13, 2011 at 10:22 PM, Philip Jägenstedt <philipj@opera.com> wrote:
> On Wed, 13 Apr 2011 13:58:04 +0200, Jack Jansen <Jack.Jansen@cwi.nl> wrote:
>
>>
>> On Apr 13, 2011, at 12:03 , Philip Jägenstedt wrote:
>>
>>> From todays teleconf.
>>>
>>> The question comes up in some test cases in
>>> <http://www.w3.org/2008/WebVideo/Fragments/TC/ua-test-cases.html>.
>>>
>>> It it my experience that media not starting at time 0 is extremely rare,
>>> I've really only seen it in poorly remuxed MPEG-2 transport streams and
>>> similar. I would say that *if* a user agent supports media that doesn't
>>> start at 0, then we should clamp the request start to the start time when
>>> necessary. I don't think that we should make the start/end relative to the
>>> media start position, as that would be inconsistent.
>>
>>
>> Everyone seems to say "we don't want to support media that doesn't start
>> at t=0", but then my next question is:
>>
>> Assuming I have an item example.ogv starting at t=0. I now request
>> <http://example.com/example.ogv?t=10,20>. Does the resulting video stream
>> start at t=0 (i.e. has everything been recoded, if the underlying format has
>> embedded timestamps) or at t=10, or do we simply leave it
>> implementation-defined?
>>
>> And, of course, the next question is: what does
>> <http://example.com.ogv?t=10,20#t=5,15> show? seconds 5-10 of the original
>> media file? seconds 10-15 of the original media file? Something else?
>>
>> Note that these examples may actually occur in real life: it's easy enough
>> to envision an application that simply tacks #t=xxxx onto an existing URL to
>> show just a little bit of it.
>
> If it in a format that allows preserving the original time stamps and the UA
> understands it, then I'd say the result is seconds 10-15 of the original
> resource. However, I've been arguing in other venues (WHATWG/HTML WG) that
> browsers in particular shouldn't support non-zero start times and should
> instead normalize the time line to start at 0. In such an implementation,
> the result would be seconds 15-20 of the original resource.
>
> So, the result depends on how the UA interprets resources with non-zero
> start times, and that is still an unresolved issue.

Ogg with Skeleton supports remembering its original start time, so it
is in theory possible to get the seconds 10-15 of the original
resource. However, I don't think I've seen it used anywhere other than
how our old annodex plugin for apache worked. So, I tend to agree that
simple practicalities of today's environments will give us the effect
that Davy has implemented. But do we have to prescribe that or leave
it or can we leave that open to be interpreted based on the resource
and the information stored within?

Silvia.

Received on Wednesday, 13 April 2011 12:51:57 UTC