[whatwg] HTML5 video: frame accuracy / SMPTE

Dirk-Willem van Gulik Dirk-Willem.van.Gulik at BBC.co.uk
Tue Jan 11 16:37:50 PST 2011

On 11 Jan 2011, at 23:00, Chris Pearce wrote:

> On 12/01/2011 11:20 a.m., Rob Coenen wrote:
>> I can imagine there are 'virtual' frames, where say frame 1 to 10 is
>> actually the same frame and internally encoded as frame 1 with a duration of
>> 10 frames?
> Yes, as I understand it, this is a legal encoding.

Though keep in mind that most decoders their output frame buffer(s) will be polled/read/pushed at a regular rate - if only to help with audio sync and flicker - and that this rate is driven/inspired/set by metadata in the encoded stream.

>> Even then I'd like the 'virtual' FPS of the WebM file exposed to the
>> webbrowser- similar to how my other utilities report a FPS.
> If the 'virtual' FPS value isn't provided by the container, and given that the frame durations could potentially have any distribution and that the media may not be fully downloaded, how can this be effectively calculated?

I cannot think of a format where this would in fact be the case - but for a few arcane ones like an animated push gif without a loop.

>> This way one could build web-tools in HTML5 that allow to access each
>> individual frame and do other things than simply playing back the movie in a
>> linear fashion from beginning to end.
> I we've discussed this sort of thing before, and roughly agreed that we'd look at processing frame data in a per-frame callback which runs in a web worker thread, if we could agree on some use cases which couldn't be achieved using SVG filters.

That sounds like a good path - so we should focus on a few use cases for just this.

> That discussion was in this thread:
> http://www.mail-archive.com/whatwg@lists.whatwg.org/msg23533.html

That is very useful. Thanks!


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