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Re: Media Fragments in Opera

From: Philip Jägenstedt <philipj@opera.com>
Date: Thu, 21 Oct 2010 11:25:54 +0200
To: public-media-fragment@w3.org
Message-ID: <op.vkw5tgansr6mfa@kirk>
On Thu, 21 Oct 2010 11:14:25 +0200, Davy Van Deursen  
<davy.vandeursen@ugent.be> wrote:

> 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
>

Having read that thread, it seems to me that SMPTE should be treated as  
labels, not as something to be converted into a timecode. In other words,  
if the resource doesn't contain these SMPTE labels, then one can't use the  
format. I'd be happy with the spec saying as much and simply not  
supporting the syntax, as neither Ogg nor WebM can embed SMPTE timecodes.

-- 
Philip Jägenstedt
Core Developer
Opera Software
Received on Thursday, 21 October 2010 09:28:36 UTC

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