Re: Media Fragments in Opera

On Thu, 21 Oct 2010 01:48:53 +0200, Silvia Pfeiffer  
<> wrote:

> On Thu, Oct 21, 2010 at 10:39 AM, Silvia Pfeiffer
> <> wrote:
>> On Thu, Oct 21, 2010 at 10:32 AM, Chris Double <>  
>> wrote:
>>> On Thu, Oct 21, 2010 at 12:24 PM, Silvia Pfeiffer
>>> <> 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
> , 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

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