[whatwg] Start position of media resources

Conrad Parker conrad at metadecks.org
Tue Apr 7 00:33:56 PDT 2009

2009/4/7 Philip Jägenstedt <philipj at opera.com>:
> On Tue, 07 Apr 2009 06:11:51 +0200, Chris Double <chris.double at double.co.nz>
> wrote:
>> On Tue, Apr 7, 2009 at 3:40 AM, Eric Carlson <eric.carlson at apple.com>
>> wrote:
>>>  Media time values are expressed in normal play time (NPT), the absolute
>>> position relative to the beginning of the presentation.
>> I don't see mention of this in the spec which is one of the reasons I
>> raised the question. Have I missed it? If not I'd like to see the spec
>> clarified here.
>> Chris.
> Indeed clarification is needed. In my opinion time 0 should correspond to
> the beginning of the media resource, no matter what the timeline of the
> container format says. This means that currentTime doesn't jump when
> playback begins and never goes beyond duration.
> Taking Ogg as an example, there's no requirement that the granulepos start
> at zero nor does a non-zero granulepos imply any special semantics such as
> "the beginning of the file has been chopped off". A tool like oggz-chop
> might retain the original granulepos of each packet or it could just as well
> adjust the stream to start at granulepos 0. Neither is more correct than the
> other, so I'd prefer we not try to infer anything from it, especially since
> such low-level timing information might be hidden deep inside the platform
> media framework (if it normalizes the time like XiphQT presumably does).
> Perhaps we would like to have some way to express where a resource is
> positioned on a timeline external to the resource itself, but could SMIL do
> this perhaps?

For Ogg, the start time of the original file (prior to chopping) is
recorded in the skeleton headers by oggz-chop, so this info is
intrinsically in the media format itself.



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