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RE: [minutes] 2012-10-10 Web Performance WG Teleconference #84

From: Jatinder Mann <jmann@microsoft.com>
Date: Thu, 11 Oct 2012 20:26:21 +0000
To: "James Robinson (jamesr@google.com)" <jamesr@google.com>, "Boris Zbarsky (bzbarsky@mit.edu)" <bzbarsky@mit.edu>, "cam@mcc.id.au" <cam@mcc.id.au>
CC: "public-web-perf@w3.org" <public-web-perf@w3.org>
Message-ID: <AE5FFD9402CD4F4785E812F2C9929D652584383C@SN2PRD0310MB383.namprd03.prod.outlook.com>
On Thu, Oct 11, 2012 at 12:46 PM, James Robinson wrote:
>> On Thu, Oct 11, 2012 at 12:40 PM, Jatinder Mann wrote:
>> In order to help synchronize multiple animations, the window.animationStartTime should return a 
>> DOMHighResTimeStamp time value at which animations started now should be considered to have 
>> started, which should be the same as the rAF callback parameter time value at that given time. This 
>> would help synchronize script animations with CSS animations, CSS transitions, and audio. The 
>> alternative would be to storing the callback parameter value in a global on every rAF callback. 
>> As window.animationStartTime was included in an earlier version of the spec and we expected this 
>> to make the spec, IE10 implements window.animationStartTime. Firefox also implements 
>> window.mozAnimationStartTime.
> The animationStartTime has never been part of a W3C specification.  For CSS animations, I do not 
> believe this is implementable in WebKit.

In Section 5, step 2 in http://www.w3.org/TR/animation-timing/, there is an editorial note:

"Editorial note
ISSUE-3 Having animation frame times run off a monotonic clock that increases at a constant rate would be better for authors than using the wallclock time as reported by the system, which might jump backwards or move forwards at varying rates due to clock slew. Doing this argues for the reinclusion of the Window.animationStartTime attribute that was present in an earlier draft, so that scripts can avoid using Date.now() to record the animation start time, which might bear little relation to the monotonic clock values anyway."

The note seems to imply that this was in the spec at some point, but that's beside the point. I think there is value in standardizing this.

Boris, Cameron, does Mozilla have any thoughts on this?

Received on Thursday, 11 October 2012 20:29:08 UTC

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