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

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.

-- 
Philip Jägenstedt
Core Developer
Opera Software

Received on Wednesday, 13 April 2011 12:23:16 UTC