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

[Darin Fisher]

Glenn summarizes my concerns exactly.  Deferred rendering is indeed the
more precise issue.

On Mon, Apr 16, 2012 at 12:18 PM, Oliver Hunt <oliver at apple.com> wrote:

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

I bet this would be fairly easy to demonstrate.


> 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.
>

You can do a lot of other things during this time.  For example, you can
prepare the next animation frame.  You can run JavaScript garbage
collection.

Also, it is common for a browser thread to handle animations for multiple
windows.  If you have animations going in both windows, it would be nice
for those animations to update in parallel instead of being serialized.

-Darin



>
> 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
>
>
>
>