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RE: Media Fragments in Opera

From: Davy Van Deursen <davy.vandeursen@ugent.be>
Date: Thu, 21 Oct 2010 11:14:25 +0200
To: "'Silvia Pfeiffer'" <silviapfeiffer1@gmail.com>, 'Philip Jägenstedt' <philipj@opera.com>
Cc: <public-media-fragment@w3.org>
Message-ID: <002501cb7100$5b72cce0$125866a0$@ugent.be>
Silvia, Philip,

> On Thu, Oct 21, 2010 at 7:31 PM, Philip Jägenstedt <philipj@opera.com> wrote:
> > On Thu, 21 Oct 2010 01:48:53 +0200, Silvia Pfeiffer
> > <silviapfeiffer1@gmail.com> wrote:
> >
> >> On Thu, Oct 21, 2010 at 10:39 AM, Silvia Pfeiffer
> >> <silviapfeiffer1@gmail.com> wrote:
> >>>
> >>> On Thu, Oct 21, 2010 at 10:32 AM, Chris Double <cdouble@mozilla.com>
> >>> wrote:
> >>>>
> >>>> On Thu, Oct 21, 2010 at 12:24 PM, Silvia Pfeiffer
> >>>> <silviapfeiffer1@gmail.com> wrote:
> >>>>>>
> >>>>>> How do you convert SMPTE to ms when you don't know the framerate?
> >>>>>> WebM doesn't provide that information.
> >>>>>
> >>>>> How do you know at what speed to display WebM files if you don't
> >>>>> have a framerate? I'm confused...
> >>>>
> >>>> We use the timestamps on the frame to know when that frame needs to
> >>>> be displayed.
> >>>
> >>> Oh, so it's possible to have a non-fixed framerate in WebM? Hmm...
> >>> Silvia.
> >>>
> >>
> >> It seems to me, looking at
> >>
> >> http://groups.google.com/a/webmproject.org/group/webm-discuss/browse_
> >> thread/thread/2f0120b464b536c1/e7e1003f1be4c4fa?lnk=st&q=smpte#e7e100
> >> 3f1be4c4fa , one could use the TimecodeScale to calculate the
> >> conversion. Since SMPTE is just a marker, it doesn't actually matter
> >> what frame rate the video is encoded in. You can convert the SMPTE
> >> time marker to an actual ms time and back only depending on what
> >> frame rate SMPTE you pick. The actual frames available don't matter
> >> since SMPTE on digital files isn't actually properly counting frames.
> >> This is why I call it a marker and not a counter.
> >
> > WebM files don't have to know their own framerate, and if it does it
> > is only informational and not actually used for playback.
> >
> > However, the SMPTE formats actually assume a framerate, taking a guess:
> >
> > smpte: 30000/1001?
> > smpte-25: 25? (if so, could be represented with 3 decimal points)
> > smpte-30: 30?
> > smpte-30-drop: 29.97? See
> > http://en.wikipedia.org/wiki/SMPTE_time_code#Drop_frame_timecode
> >
> > What does one do when the framerate assumed by the format doesn't
> > match the resource, or the framerate of the resource isn't known?
> 
> It doesn't matter. You get a SMPTE timecode which is just a label for a time offset. Whether there is an actual frame at that time
offset
> or whether there is a frame slightly earlier is something you cannot control. You will only get a frame at exactly that time if
you are
> using the SMPTE time code that has a frame rate that equals your video's framerate. Don't regard SMPTE as a counter, but only as a
> marker, i.e. a label for a time offset. The SMPTE folks had that discussion with me about 10 years ago. ;-)

I think we had a similar discussion already on this mailinglist, see [1] for the initial mail of the thread.

Best regards,

Davy

[1] http://lists.w3.org/Archives/Public/public-media-fragment/2009Jan/0013.html 

-- 
Davy Van Deursen

Ghent University - IBBT
Department of Electronics and Information Systems - Multimedia Lab
URL: http://multimedialab.elis.ugent.be/dvdeurse
Received on Thursday, 21 October 2010 09:15:09 UTC

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