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[whatwg] What is the purpose of timeupdate?

From: Andrew Scherkus <scherkus@chromium.org>
Date: Thu, 5 Nov 2009 12:11:15 -0800
Message-ID: <eb35a2a80911051211n47e60418oc4c864e16238a27e@mail.gmail.com>
On Thu, Nov 5, 2009 at 6:10 AM, Brian Campbell <
brian.p.campbell at dartmouth.edu> wrote:

> 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.
> -- Brian

I'll see if we can do something for WebKit based browsers, because today it
literally is hardcoded to 250ms for all ports.

Maybe we'll end up firing events based on frame updates for video, and
something arbitrary for audio (as it is today).

Brian, since Firefox is doing what you proposed -- can you think of any
other issues with its current implementation?  What about for audio files?

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