[whatwg] [canvas] request for {create, get, put}ImageDataHD and ctx.backingStorePixelRatio

Maciej Stachowiak mjs at apple.com
Mon Apr 16 14:06:20 PDT 2012

On Apr 16, 2012, at 12:10 PM, Glenn Maynard wrote:

> On Mon, Apr 16, 2012 at 1:59 PM, Oliver Hunt <oliver at apple.com> wrote: 
> I don't understand why adding a runloop cycle to any read seems like something that would introduce a much more noticable delay than a memcopy.
> The use case is deferred rendering.  Canvas drawing calls don't need to complete synchronously (before the drawing call returns); they can be queued, so API calls return immediately and the actual draws can happen in a thread or on the GPU.  This is exactly like OpenGL's pipelining model (and might well be implemented using it, on some platforms).
> The problem is that if you have a bunch of that work pipelined, and you perform a synchronous readback, you have to flush the queue.  In OpenGL terms, you have to call glFinish().  That might take long enough to cause a visible UI hitch.  By making the readback asynchronous, you can defer the actual operation until the operations before it have been completed, so you avoid any such blocking in the UI thread.
>  I also don't understand what makes reading from the GPU so expensive that adding a runloop cycle is necessary for good perf, but it's unnecessary for a write.
> It has nothing to do with how expensive the GPU read is, and everything to do with the need to flush the pipeline.  Writes don't need to do this; they simply queue, like any other drawing operation.

Would the async version still require a flush and immediate readback if you do any drawing after the get call but before the data is returned?


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