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

[Oliver Hunt]

On Apr 16, 2012, at 1:12 PM, Darin Fisher <darin at chromium.org> wrote:

> 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.

Then by all means do :D

> 
> 
> 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.

None of which changes the fact that your actual developer now needs more complicated code, and has slower performance.  If I'm doing purely imagedata based code then there isn't anything to defer, and so all you're doing is adding runloop latency.  The other examples you give don't really apply either. 

Most imagedata both code i've seen is not GC heavy, if you're performing animations using css animations, etc then I believe that the browser is already able to hoist them onto another thread.  If you have animations in multiple windows then chrome doesn't have a problem because those windows are a separate process, and if you're not, then all you're doing is allowing one runloop of work (which may or may not be enough to get a paint done) before you start processing your ImageData.  I'm really not sure what it is that you're doing with your ImageData such that it takes so much less time than the canvas work, but it seems remarkable that there's some operation you can perform in JS over all the data returned that takes less time that the latency introduced by an async API.

--Oliver

> 
> -Darin
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>  
> 
> 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
>> 
>> 
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