[whatwg] Start position of media resources

[Silvia Pfeiffer]

On Tue, Apr 7, 2009 at 5:12 PM, Philip Jägenstedt <philipj at opera.com> wrote:
> 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.

I humbly disagree. If a media file explicitly knows at what time
offset it starts, the timeline needs to represent that, otherwise
there are contradictory user experiences.

For example, take a video that is a subpart of a larger video and has
been delivered through a media fragment URI
(http://www.w3.org/2008/WebVideo/Fragments/WD-media-fragments-reqs/).
When a user watches both, the fragment and the full resource, and both
start at 0, he/she will assume they are different resources, when in
fact one is just a fragment of the other. Worse still: if he/she tries
to send a URI with a link to a specific time offset in the video to a
friend, he/she will come up with diverging URIs based on whether
he/she watched the fragment or the full resource. Representing the
wrong timeline for a media fragment will only cause confusion and
inconsistencies.


> 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).

For Ogg and the definition of Skeleton
(http://wiki.xiph.org/index.php/Ogg_Skeleton), both the original
basetime of the beginning of the file and the presentation time of the
chopped off part are recorded, so it actually does imply special
semantics.


Regards,
Silvia.