[whatwg] What is the purpose of timeupdate?
brian.p.campbell at Dartmouth.EDU
Thu Nov 5 06:10:17 PST 2009
On Nov 5, 2009, at 1:17 AM, Andrew Scherkus wrote:
> On Fri, Oct 30, 2009 at 10:18 PM, Brian Campbell <Brian.P.Campbell at dartmouth.edu
> > wrote:
> As a multimedia developer, I am wondering about the purpose of the
> timeupdate event on media elements. On first glance, it would appear
> that this event would be useful for synchronizing animations,
> bullets, captions, UI, and the like. The spec specifies a rate of 4
> to 66 Hz for these events. The high end of this (30 or more Hz) is
> pretty reasonable for displaying things in sync with the video. The
> low end, however, 4 Hz, is far too slow for most types of
> synchronization; everything feels laggy at this frequency. From my
> testing on a two year old MacBook Pro, Firefox is giving me about 25
> timeupdate events per second, while Safari and Chrome are giving me
> the bare minimum, of 4 timeupdate events per second.
> At 4 timeupdate events per second, it isn't all that useful. I can
> replace it with setInterval, at whatever rate I want, query the
> time, and get the synchronization I need, but that makes the
> timeupdate event seem to be redundant. At 25 timeupdate events per
> second, it is reasonably useful, and can be used to synchronize
> various things to the video.
> So, I'm wondering if there's a purpose for the timeupdate event that
> I'm missing. If it is intended for any sort of synchronization with
> the video, I think it should be improved to give better guarantees
> on the interval between updates, or just dropped from the spec; it's
> not useful enough in its current form. To improve it, the maximum
> interval between updates could be reduced to about 40 ms, or perhaps
> the interval could be made settable so the author could control how
> often they want to get the event.
> -- Brian
> I believe it's a convenience over using setTimeout/setInterval +
> polling to determine whether playback has progressed ie., for
> rendering your own playback progress bar. I've also seen it been
> used as a signal to copy frames into <canvas> on Firefox, however if
> timeupdate frequency != fps of video you either miss frames or paint
> too much.
> I don't think timeupdate today is very useful for doing anything
> beyond a progress bar or other simple synchronized animations.
Right. That's what I figured the point is; I just wanted to check to
make sure I wasn't missing something.
As implemented by Safari and Chrome (which is the minimum rate allowed
by the spec), it's not really useful for that purpose, as 4 updates
per second makes any sort of synchronization feel jerky and laggy. If
it were done at the frame rate of the video, or perhaps with a minimum
of 25 frames per second, it would be much more useful. Even at a
minimum of 15 frames per second, you would still be able to get some
sorts of useful synchronization, though animations synchronized
wouldn't feel as smooth as they could.
> Would something like <video> firing events for every frame rendered
> help you out? This would help also fix the <canvas> over/under
> painting issue and improve synchronization.
Yes, this would be considerably better than what is currently specced.
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