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

[Oliver Hunt]

Could someone construct a demonstration of where the read back of the imagedata takes longer than a runloop cycle?

You're asking for significant additional complexity for content authors, with a regression in general case performance, it would be good to see if it's possible to create an example, even if it's not something any sensible author would do, where their is a performance improvement.

Remember, the application is only marginally better when it's not painting due to waiting for a runloop cycle than it is when blocked waiting on a graphics flush.

Also, if the argument is wrt deferred rendering rather than GPU copyback, can we drop GPU related arguments from this thread?

--Oliver

On Apr 16, 2012, at 12:10 PM, Glenn Maynard <glenn at zewt.org> 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.
> 
> -- 
> Glenn Maynard
> 
>