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Re: [whatwg] <canvas> feedback

From: K. Gadd <kg@luminance.org>
Date: Wed, 14 May 2014 19:30:08 -0700
Message-ID: <CAPJwq3XpZEBB-Dh4nUmKfJxhZfaA-oteMOpDk68Qj9CnjafehQ@mail.gmail.com>
To: Jürg Lehni <lists@scratchdisk.com>
Cc: WHAT Working Group <whatwg@whatwg.org>, Ian Hickson <ian@hixie.ch>
Is it ever possible to make canvas-to-canvas blits consistently fast?
It's my understanding that browsers still make
intelligent/heuristic-based choices about which canvases to
accelerate, if any, and that it depends on the size of the canvas,
whether it's in the DOM, etc. I've had to report bugs related to this
against firefox and chrome in the past, I'm sure more exist. There's
also the scenario where you need to blit between Canvas2D canvases and
WebGL canvases - the last time I tried this, a single blit could cost
*hundreds* of milliseconds because of pipeline stalls and cpu<->gpu
transfers.

Canvas-to-canvas blits are a way to implement layering, but it seems
like making it consistently fast via canvas-canvas blits is a much
more difficult challenge than making sure that there are fast&cheap
ways to layer separate canvases at a composition stage. The latter
just requires that the browser have a good way to composite the
canvases, the former requires that various scenarios with canvases
living in CPU and GPU memory, deferred rendering queues, etc all get
resolved efficiently in order to copy bits from one place to another.

(In general, I think any solution that relies on using
canvas-on-canvas drawing any time a single layer is invalidated is
suspect. The browser already has a compositing engine for this that
can efficiently update only modified subregions and knows how to cache
reusable data; re-rendering the entire surface from JS on change is
going to be a lot more expensive than that. Don't some platforms
actually have compositing/layers at the OS level, like CoreAnimation
on iOS/OSX?)

On Wed, May 14, 2014 at 6:30 AM, Jürg Lehni <lists@scratchdisk.com> wrote:
> On Apr 30, 2014, at 00:27 , Ian Hickson <ian@hixie.ch> wrote:
>
>> On Mon, 7 Apr 2014, Jürg Lehni wrote:
>>>
>>> Well this particular case, yes. But in the same way we allow a group of
>>> items to have an opacity applied to in Paper.js, and expect it to behave
>>> the same ways as in SVG: The group should appear as if its children were
>>> first rendered at 100% alpha and then blitted over with the desired
>>> transparency.
>>>
>>> Layers would offer exactly this flexibility, and having them around
>>> would make a whole lot of sense, because currently the above can only be
>>> achieved by drawing into a separate canvas and blitting the result over.
>>> The performance of this is real low on all browsers, a true bottleneck
>>> in our library currently.
>>
>> It's not clear to me why it would be faster if implemented as layers.
>> Wouldn't the solution here be for browsers to make canvas-on-canvas
>> drawing faster? I mean, fundamentally, they're the same feature.
>
> I was perhaps wrongly assuming that including layering in the API would allow the browser vendors to better optimize this use case. The problem with the current solution is that drawing a canvas into another canvas is inexplicably slow across all browsers. The only reason I can imagine for this is that the pixels are copied back and forth between the GPU and the main memory, and perhaps converted along the way, while they could simply stay on the GPU as they are only used there. But reality is probably more complicated than that.
>
> So if the proposed API addition would allow a better optimization then I'd be all for it. If not, then I am wondering how I can get the vendor's attention to improve this particular case. It really is very slow currently, to the point where it doesn't make sense to use it for any sort of animation technique.
>
> J
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Received on Thursday, 15 May 2014 02:31:24 UTC

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