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[whatwg] What is the purpose of timeupdate?

From: Andrew Scherkus <scherkus@chromium.org>
Date: Wed, 4 Nov 2009 22:17:09 -0800
Message-ID: <eb35a2a80911042217kb20a05fg4259ea4d9ed4d219@mail.gmail.com>
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.

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.

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