[whatwg] HTML5 video: frame accuracy / SMPTE

Dirk-Willem van Gulik Dirk-Willem.van.Gulik at BBC.co.uk
Tue Jan 11 16:58:28 PST 2011

On 12 Jan 2011, at 00:48, Dirk-Willem van Gulik wrote:

> the clock relative to shutter/gating to the end user - as this is what you need to avoid flicker, interlace issues, half the frame showing > the next scene, etc.

Apologies - got some private mail asking for examples. So the simplest example I can think of is a second of black video followed by a second of white. At the moment of transition - the creative person designing this wanted it to perfectly 'flash' from back to white.

If somehow you updated the display halfway a refresh cycle (and lets assume your update process happens from top to bottom) then for 'one refresh cycle' you'd show a black (old) top half and a white bottom half. You can get similar fun and games with objects moving faster (or around) the speed of your shown-as-size/update cycle. And it gets more complex when your 'screen' does not update in a simple way - but uses interlacing[1].

Now in practice we generally avoid this by having double buffer, slaving our frame refresh to the video card or in the video card to something else, etc. But it is easy to get wrong.



1: http://en.wikipedia.org/wiki/Interlaced_video - try not to get a headache !

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