API Review RT:17407 Canvas Node

Kevin Rushforth kevin.rushforth at oracle.com
Wed Apr 25 13:16:18 PDT 2012

Right, the "drawing" commands just write into an internal buffer. The 
buffer is synced to Prism along with any scene graph changes, and the 
actual rendering happens on the rendering thread along with any other 
rendering. Given this, I wouldn't expect the UI thread to be blocked for 
any appreciable length of time just filling up the buffer.

If it does become a problem, we could probably extend the API to support 
creating a GC and then setting it on the Canvas as you suggest. Or do 
you think that is required for the first version?

-- Kevin

Richard Bair wrote:
> Why can't I draw on a background thread? Don't we internally setup a buffer of draw commands, and then sync those across during the pulse synchronization? If so, can't we let people draw on a background thread (maybe to a created GC?) and then set that on the Canvas in one shot on the FX thread?
> This does seem like a pretty compelling use case. Especially since we won't be handling dirty regions for anybody, they might have a fair amount to draw.
> Richard
> On Apr 25, 2012, at 11:23 AM, Jim Graham wrote:
>> Right now when you draw it has to be on the thread on which you respond to inputs or do other SG modifications.  In other words, any thread where you can appropriately call cv.setWidth(), group.add(cv), etc. is a thread you can render on.  The rendering is "remembered" in a buffer and it isn't actually drawn until the render pass.  At that point we simply eat the buffer and render it in a tight loop so any pauses for thought in your rendering code would only affect the event and SG modification threads, not the render thread.  (The pauses aren't stored in the buffer. ;)
>> We looked at ways of relaxing that further, but they won't come in this first round of the API.  It would necessitate having multiple HW contexts and have a context set aside to only be used for Canvas rendering into its texture with a clearly defined buffer swap that has to be synchronized across the threads.  Unfortunately that model is not easy to support with D3D, our most common hw solution, so it is still just a pet theory...
>> 			...jim
>> On 4/25/12 1:38 AM, Dr. Michael Paus wrote:
>>> Am 25.04.2012 10:03, schrieb Jim Graham:
>>>> Hmmm... More variants of Canvas or more attributes to set...?
>>> I was just thinking about potential use-cases for the canvas node and
>>> stumbled over the
>>> following question. How can I still provide a super-smooth GUI if I have
>>> a canvas drawing
>>> which lasts longer than the interval between two pulses. Maybe it even
>>> lasts more than
>>> a second. My understanding of the current canvas design proposal is that
>>> when I redraw
>>> the canvas, for whatever reason, the GUI is basically blocked for this
>>> time. Is that true?
>>> If the above interpretation is right I would like to propose a
>>> modification of the design
>>> which makes it possible to have a double buffer canvas where the
>>> currently active
>>> buffer is redrawn whenever the JavaFX runtime needs to redraw the node
>>> and where
>>> I can render into the second buffer from ANY thread at any time without
>>> disturbing the
>>> smoothness of the GUI. When I am done with the rendering I just signal a
>>> buffer switch
>>> and with the next pulse the new buffer is shown.
>>> What do you think?
>>> Michael

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