[whatwg] HTML5 video: frame accuracy / SMPTE

Chris Pearce chris at pearce.org.nz
Tue Jan 11 17:43:12 PST 2011

On 12/01/2011 2:22 p.m., Dirk-Willem van Gulik wrote:
> On 12 Jan 2011, at 01:17, Chris Pearce wrote:
>>> 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.
>> WebM can be variable frame rate. At best the WebM container specification [http://www.webmproject.org/code/specs/container/#track] lists the FrameRate block as "Informational only", which presumably means the value stored in the container can't be trusted.
> Right - but is there a WebM decoder which is able to hand it off that way ? AFAIK they all use that value or select a default/measured rounded heuristic to solve flicker ?

Firefox 4 doesn't use the frame rate stored in the container for WebM. 
Each frame is stored with its presentation time, and we request repaints 
as each frame fall due for painting. The prevention of flicker is 
handled by our graphics layer, video doesn't really participate in that, 
it just hands off frames downstream when they're due for painting. We 
have plans to schedule video frame painting more preemptively in future, 
but I imagine we'd still use the presentation time encoded with each 
frame when we do that.

Chris P.

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