On Tue, 07 Apr 2009 10:26:15 +0200, Silvia Pfeiffer
<silviapfeiff...@gmail.com> wrote:
On Tue, Apr 7, 2009 at 5:12 PM, Philip Jägenstedt <phil...@opera.com>
wrote:
On Tue, 07 Apr 2009 06:11:51 +0200, Chris Double
<chris.dou...@double.co.nz>
wrote:
On Tue, Apr 7, 2009 at 3:40 AM, Eric Carlson <eric.carl...@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.
If the media resource really does explicitly define an offset then I might
agree that it makes sense to adjust the timeline.
However, for plain Ogg or any other container format that just happens to
have a non-zero timestamp at the beginning of the file I think we should
definitely align them to zero. You can get such files easily by cutting
streams and it would be confusing if the timeline was relative to the
original file. As an example, in MPEG the PTS (Presentation Time Stamp)
can start at non-zero, be discontinuos and wrap around 0 so normalization
is necessary. I'm not sure if anyone disagrees, but it would be a very bad
idea to infer any semantics from the container time stamps in the absense
of some explicit mapping like Ogg Skeleton.
Not generally requiring low-level inspection of the container format time
stamps is important to us as that would virtually require that the UA
itself to demux and inspect the time stamps of different container
formats. If a platform media framework is used, time is normalized in some
way, likely differently depending on framework and plugins used.
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.
OK, I agree with this.
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.
The most consistent behavior in my opinion would be to report duration as
the duration of the whole resource and for buffered/played/seekable to
only return ranges within the range indicated by the media fragment.
Exposing the start and stop position of the fragment via the DOM seems
like overkill to me at this point.
--
Philip Jägenstedt
Opera Software
