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Re: Media Fragments in Opera

From: Philip Jägenstedt <philipj@opera.com>
Date: Thu, 21 Oct 2010 10:31:03 +0200
To: public-media-fragment@w3.org
Message-ID: <op.vkw2911jsr6mfa@kirk>
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#e7e1003f1be4c4fa
> , 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?

-- 
Philip Jägenstedt
Core Developer
Opera Software
Received on Thursday, 21 October 2010 08:31:43 UTC

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