Re: [w3ctag/design-reviews] Review OffscreenCanvas, including ImageBitmapRenderingContext (#141)

It looks like when requestAnimationFrame was added to workers (which is still in progress in https://github.com/whatwg/html/pull/3677 ), the behavior chosen was to automatically commit frames for OffscreenCanvas contexts which are modified in the rAF callback. This behavior was probably chosen in order to behave most similarly to rAF on the main thread.

In this definition it's an error to call commit() from inside a rAF callback on a worker. The spec and implementations should be updated to throw an exception from commit() in this case, to make it clearer.

Thanks for putting together your examples. Per your Chrome bug report http://crbug.com/859275 , here's a revised version of your resizing example which removes the call to commit() and which works well:

http://jsfiddle.net/gmpkvu30/2/

and a revised version of your 15 FPS animation example which removes the call to commit() and which works well:

http://jsfiddle.net/y36aujrf/3/

commit() can usefully be used within setTimeout() callbacks on a worker, and also within WebAssembly threads, where it's basically the only way to get frames out for display. Yes, in this case events aren't processed on the thread, but WebAssembly threads will expect to use the heap's SharedArrayBuffer to transfer data in and out.

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