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Re: exposing CANVAS or something like it to Web Workers

From: Kenneth Russell <kbr@google.com>
Date: Wed, 20 Feb 2013 11:43:06 -0800
Message-ID: <CAMYvS2dVuXurUOPFAWhEr6Y=_bbOUoNBmdfn8U4JJnU63pHn0w@mail.gmail.com>
To: Travis Leithead <travis.leithead@microsoft.com>
Cc: Gregg Tavares <gman@google.com>, Ian Hickson <ian@hixie.ch>, Charles Pritchard <chuck@jumis.com>, Web Applications Working Group WG <public-webapps@w3.org>
On Fri, Feb 8, 2013 at 9:14 AM, Travis Leithead
<travis.leithead@microsoft.com> wrote:
>>> What would be the advantage? If you wanted to keep dom elements in sync
>>> with the canvas you'd still have to post something from the worker back to
>>> the main thread so the main thread would know to pop.
>
>
>
> Well, it's not a fleshed out proposal by any stretch, but you could imagine
> that an event could be used to signal that new frames were ready from the
> producer—then the main thread would know to pop.

It sounds like this approach would require a bunch of new concepts --
a remote worker context, events signalled through it, etc. It would
still be necessary to postMessage from the main thread back to the
worker for flow control. Flow control is definitely necessary -- the
producer can't just produce frames without any feedback about when
they're actually consumed. We've had problems in Chrome's graphics
pipeline in the past where lack of flow control led to slow and
inconsistent frame rates.

I'm excited about Gregg's proposal because it solves a lot of use
cases that aren't currently addressed by CanvasProxy, using a couple
of simple primitives that build upon others already in the platform
(cross-document messaging and Transferables).

How can we move forward so that user agents can experimentally
implement these APIs? Ideally we'd prototype and experiment with them
in some medium-sized applications before finalizing any specs in this
area.

Thanks,

-Ken


> From: Gregg Tavares [mailto:gman@google.com]
> Sent: Friday, February 8, 2013 3:14 AM
> To: Travis Leithead
> Cc: Ian Hickson; Charles Pritchard; Web Applications Working Group WG
>
>
> Subject: Re: exposing CANVAS or something like it to Web Workers
>
>
>
>
>
>
>
> On Thu, Feb 7, 2013 at 10:46 PM, Travis Leithead
> <travis.leithead@microsoft.com> wrote:
>
> Having thought about this before, I wonder why we don’t use a
> producer/consumer model rather than a transfer of canvas ownership model?
>
>
>
> A completely orthogonal idea (just my rough 2c after reading Gregg’s
> proposal), is to have an internal frame buffer accessible via a WorkerCanvas
> API which supports some set of canvas 2d/3d APIs as appropriate, and can
> “push” a completed frame onto a stack in the internal frame buffer. Thus the
> worker can produce frames as fast as desired.
>
>
>
> On the document side, canvas gets a 3rd kind of context—a
> WorkerRemoteContext, which just offers the “pop” API to pop a frame from the
> internal frame buffer into the canvas.
>
>
>
> Then you just add some basic signaling events on both ends of the frame
> buffer and you’re good (as far as synchronizing the worker with the
> document). The producer (in the worker) is free to produce multiple frames
> in advance (if desired), while the consumer is able to pop frames when
> available. You could even have the framebuffer depth configurable.
>
>
>
> What would be the advantage? If you wanted to keep dom elements in sync with
> the canvas you'd still have to post something from the worker back to the
> main thread so the main thread would know to pop.
>
>
>
>
>
>
>
> From: Gregg Tavares [mailto:gman@google.com]
> Sent: Thursday, February 7, 2013 2:25 PM
> To: Ian Hickson
> Cc: Charles Pritchard; Web Applications Working Group WG
> Subject: Re: exposing CANVAS or something like it to Web Workers
>
>
>
> I put up a new proposal for canvas in workers
>
>
>
> http://wiki.whatwg.org/wiki/CanvasInWorkers
>
>
>
> Please take a look.
>
>
>
> This proposal comes from offline discussions with representatives from the
> various browsers as well as input from the Google Maps team. I can post a
> summary here if you'd like but it might be easier to read the wiki
>
>
>
> Looking forward to feedback.
>
>
>
>
>
>
>
> On Tue, Jan 8, 2013 at 10:50 AM, Ian Hickson <ian@hixie.ch> wrote:
>
> On Wed, 2 Jan 2013, Gregg Tavares (社ç~T¨) wrote:
>>
>> Another issue as come up and that is one of being able
>> to synchronize updates of a canvas in
>> worker with changes in the main page.
>
> For 2D, the intended solution is to just ship the ImageBitamp from the
> worker canvas to the main thread via a MessagePort and then render it on
> the canvas at the appropriate time.
>
> I don't know how you would do it for WebGL.
>
>
>
>> Similarly, let's say you have 2 canvases and are rendering to both in a
>> worker.  Does
>>
>>    context1.commit();
>>    context2.commit();
>>
>> guarantee you'll see both commits together?
>
> No, unfortunately not. There's no synchronisation between workers and the
> main thread (by design, to prevent any possibility of deadlocks), and
> there's not currently a batching API.
>
> However, if this becomes a common problem (which we can determine by
> seeing if we get bugs complaining about different parts of apps/games
> seeming to slide around or generally be slightly out of sync, or if we see
> a lot of authors shunting multiple ImageBitmap objects across MessagePort
> channels) we can always add an explicit batching API to make this kind of
> thing easy.
>
> Note that in theory, for 2D at least, shunting ImageBitmaps across threads
> can be as efficient as commit().
>
>
> --
> Ian Hickson               U+1047E                )\._.,--....,'``.    fL
> http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
> Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
>
>
>
>
Received on Wednesday, 20 February 2013 19:43:33 GMT

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