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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 19:39:48 +0000
To: "James Robinson (jamesr@google.com)" <jamesr@google.com>, "public-web-perf@w3.org" <public-web-perf@w3.org>
Message-ID: <AE5FFD9402CD4F4785E812F2C9929D652584366B@SN2PRD0310MB383.namprd03.prod.outlook.com>
On Thu, Oct 11, 2012 at 11:27 PM, James Simonsen wrote:
>> On Wed, Oct 10, 2012 at 12:51 PM, Jatinder Mann wrote: 
>> -          ISSUE-5: Expected callback rates should be documented, http://www.w3.org/2010/webperf/track/issues/5 
>> There had been feedback to the working group that browsers have different behaviors on the expected callback rate 
>> when the page is not visible; Firefox and Chrome throttle callbacks, IE does not issue callbacks. The spec needs to be 
>> clear on whether we want to have a specific definition on the expected behavior or specifically call out that this is 
>> implementation specific behavior.
>
> The processing model requires that callbacks not fire when the page is not visible.  See the first sentence of 
> "http://dvcs.w3.org/hg/webperf/raw-file/tip/specs/RequestAnimationFrame/Overview.html#processingmodel".  
> Chrome does not issue callbacks at all when a page is not visible, as the spec requires.  I don't think we should make 
> this implementation specific.
 
I'm glad the spec is very precise on this point and that it defines the more efficient option. There was some confusion in the conf call yesterday where some folks thought the spec wasn't clear. Thanks for clearing this up.


> On Wed, Oct 10, 2012 at 12:51 PM, Jatinder Mann wrote:
> -          ACTION-31: Consider including a window.animationStartTime and use High Resolution Time for the rAF callback 
> parameter, http://www.w3.org/2010/webperf/track/actions/31 
> The second request in this action, updating the rAF callback parameter to use DOMHighResTimeStamp instead of 
> DOMTimeStamp, has already been completed in the latest editor's draft. The issue to include 
> window.animationStartTime in the specification will be discussed on the mailing list.

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.

I would like to see this added to the spec.

I believe those were the only remaining issues on this spec. 

Thanks,
Jatinder
Received on Thursday, 11 October 2012 19:41:48 GMT

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