[whatwg] Counterproposal for canvas in workers

Rik Cabanier cabanier at gmail.com
Thu Oct 17 21:50:43 PDT 2013

Extra methods on the canvas API:

Promise setTaskScript(DOMString script); // can error be in promise?
Promise executeTask(DOMString id, dictionary json, boolean synchronized =
true); // Transferable elements allowed in dictionary

Object that is active in the task:

interface CanvasTask {

HTMLCanvasElement createCanvas(unsigned long width, unsigned long height);
attribute Function onTask;


CanvasTask implements HTMLCanvasElement;

Example code:

var c = document.getElementById("gameCanvas");

var gameState = {};

window.addEventListener("load", function(){


c.executeTask("mainscene", gameState);




c.executeTask("mainscene", gameState);


Example code for gameLogic.js:

var ctx = getContext("2d");

onTask = function(DOMString id, dictionary json) {

if(id == "mainscene") {

if(typeof(MinionCanvas)=="Undefined") {

MinionCanvas = createCanvas(200, 300);

MinionCanvas.executeTask("drawMinion", {}) // creates promise under the hood



SpaceShipCanvas = createCanvas(300, 300);

SpaceShipCanvas.executeTask("drawSpaceShip", gameState); // redraw spaceship

executeTask("drawBackDrop",  gameState); // in other task

executeTask("drawBoss", gameState); // lots of js to draw the boss so
better done in task

for(...) //for each minion {

... // set the matrix
ctx.drawImage(MinionCanvas, ...); // draw the minion <- note that the
minion might still be drawing in the other thread

for(...) //for each spaceship {

..// set the matrix

ctx.drawImage(SpaceShipCanvas); // draw the spaceship <- it might still be
drawing in the other task


.. // other drawing commands for score, controls, etc

} else if(id == "drawMinion") {


}  else if(id == "drawSpaceShip") {

... // set up tasks to draw parts of the ship?

} ...


On Thu, Oct 17, 2013 at 8:10 PM, Rik Cabanier <cabanier at gmail.com> wrote:

> On Thu, Oct 17, 2013 at 4:01 PM, Robert O'Callahan <robert at ocallahan.org>wrote:
>> On Fri, Oct 18, 2013 at 10:56 AM, Justin Novosad <junov at google.com>wrote:
>>> On Thu, Oct 17, 2013 at 5:50 PM, Rik Cabanier <cabanier at gmail.com>wrote:
>>>> Creating temporary canvases is still possible. I'm unsure how it would
>>>> be different from a worker.
>>>> An advantage would be that you can draw to the temporary canvases in
>>>> parallel to using them. Only PIXEL access is disallowed, you can still call
>>>> drawImage using a canvas that has outstanding tasks.
>>> Right. The write-only restriction would only apply to canvas contexts
>>> that commit (push their tasks) directly down to the compositor. You could
>>> still create a canvas that is local to the worker, rasterize it in the
>>> worker and do readbacks in the worker, create ImageBitmaps from it, etc.
>> I'm not sure that you and Rik are talking about the same thing, since
>> he's still talking about "outstanding tasks". If you are talking about the
>> same thing, I don't know what it is. I'd like to see some concrete details
>> for what you'd change in the current WorkerCanvas proposal. For the sake of
>> clarity I've put (my understand of) it here:
>> https://wiki.mozilla.org/User:Roc/WorkerCanvasProposal
> I'll work on drawing up an example of my proposal.
> With WorkerCanvas and transferToImageBitmap, you can draw multiple layers
>> in parallel (and actually draw, not just queue drawing commands) by
>> creating multiple workers, having them each produce an ImageBitmap, and
>> compositing those ImageBitmaps together by stacking <img> elements or
>> drawing them all to a single canvas. It uses more memory but you get more
>> parallelism.
> They would still have to wait for each other so the images are composited
> in-order. If you don't care about that, the 'synchronized' option would let
> you draw as soon as you exit the task (which is how Chrome always draws
> since it's faster)
> In fact, an implementation could choose to take the deferred-drawing
>> approach instead. You would queue up drawing commands in the WorkerCanvas
>> (or the drawing context), and then transferToImageBitmap would not
>> immediately render but produce an ImageBitmap implementation encapsulating
>> the list of drawing commands to be drawn later, wherever/whenever that
>> ImageBitmap ended up being used. I think for commit() the implementation
>> would always want to force rasterization on the worker (or possibly some
>> dedicated canvas-rendering thread); you could forward a list of drawing
>> commands to the compositor thread for rasterization but I don't think
>> there's any reason to do that (and some good reasons not to).
> Can you tell me how you can ensure that you don't do too much work?
> Drawing in a continuous loop using 'Commit' would waste a lot of resources.

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