Blur effect on live scene?

Kevin Rushforth kevin.rushforth at
Thu Aug 13 19:33:04 UTC 2015

Seems there are a few different issues here. I'll weigh in on a couple 
of them. To summarize what Jim has already said:

* We always do GPU hardware rendering wherever possible

* Node caching was intended as a hint to the system to cache the result 
of potentially expensive effects into a texture (which is rendered into 
by the GPU) and re-render that texture when needed. Jim was suggesting 
that an application might be able to use that mechanism to do something 
other than its original purpose.

* Snapshot was intended to allow an application to take (as the name 
suggests) a snapshot of a scene or node rendered into an image and 
return that image to the application. We don't use snapshot internally, 
rather various "render and then get the results" operations like 
snapshot, node caching, and rendering to a JFXPanel all use similar 
mechanisms which boil down to "render to texture" via OpenGL or D3D. We 
have considered providing new API to allow more of a "live 
render-to-texture" operation in the future, but haven't thought about it 

Much of the discussion has been centered on what is possible with the 
current JavaFX API, but there has also been some discussion on what 
might be done with a new API.

Anyway, to comment on a couple specific points, I have some inline 
comments below.

Jim Graham wrote:
> On 8/13/2015 1:29 AM, Matthias Hänel wrote:
>>>>> I'd argue that we sort of do have something like that - it is the 
>>>>> cache flag.  If a Node is cached then we do have a copy of it in a 
>>>>> texture and that can help make the Blur Effect work more 
>>>>> efficiently, but there may be some additional copies between 
>>>>> textures if everything isn't set up right.  Still, that is an 
>>>>> avenue for someone to check to see if there isn't a better way to 
>>>>> achieve this effect in the short term...
>>>> I am not pretty sure what cache does. Probably some hasmap that 
>>>> holds objects and they are not instantly destroyed in the graphics 
>>>> RAM?
>>> It is not a hashmap.
>>> It is a hint to save the rendering of that node in a buffer:
>>> I don't like the way that this doc comment is worded as it implies 
>>> that using it on a node that is blurred is unwise, but if the node 
>>> is animated over even GPU acceleration of the rendering and blurring 
>>> operations" are going to have some cost that it could save.
>> That documentation says nodes are cached as Bitmaps. In GPU or in CPU 
>> space? It is not clear here. I suspect it is in CPU RAM space.
>> That will just lead to more copy-tasks from CPU to GPU or not? If 
>> optimized there is no benefit at all as stated in the docu "note that 
>> on some platforms such as GPU accelerated platforms there is little 
>> benefit".
> As I said, the doc comments there are confusing.  "bitmap" here, 
> despite any capitalization, is a general term for "some type of pixel 
> store". On GPU accelerated platforms, this is a vram texture/FBO.
>> Actually, I would expect that renered Nodes are textures in GPU-VRAM 
>> to get the most performance out of it.
> Yes, nodes with the cache hint are rendered into VRAM textures and 
> reused for subsequent frames if the cache hint is set - subject to the 
> policies of the cacheHint property if any (mainly transform) 
> properties of the node are changed in the meantime.
>>>>  From my current point the major problem with JavaFX is still the 
>>>> same.
>>>> 1. Has a good API
>>>> 2. renders most of its stuff in software, hence does not run 
>>>> performant
>>>> 3. Has good approaches, but the overall sight on the technology is 
>>>> broken somewhere.
>>> I am not sure how you come to the conclusion that it renders most of 
>>> its stuff in software.  It renders quite a lot in hardware.  Even 
>>> the example here of using snapshot to optimize a blurred background 
>>> - the rendering of the scene is done in hw.  It is only copied to 
>>> main memory because the API requires a persistent image.  If you 
>>> render that image to the screen it is copied back into a texture and 
>>> reused from that texture unless we run low on vram.  There is no 
>>> rendering in software there, only use of a heap buffer for 
>>> persistent storage...
>> Your point is that it uses hardware to render and it just uses 
>> snapshot to satisfy the API?
> What is "it"?
> JavaFX doesn't "use" snapshot anywhere.  It provides the snapshot API 
> so the developer can use snapshot where the developer wants to, but 
> JavaFX doesn't invoke the snapshot API on its own.
> In particular, the node cache hint does not use the snapshot API under 
> the covers.
>> My current understaning is ... what happens in our blurred-effect case?
> To be clear, you are describing what happens in a particular 
> implementation of the blurred-effect case.  The implementation that 
> uses snapshot, which may be the popular technique in use at this time.
>> 1. the application constructs a JavaFX node tree.
>> 2. the node tree is rendered mostly in hardware (shader effects and 
>> so on on top) to the main framebuffer
> Correct
>> 3. Snapshot calls ReadPixels (or whatever it is called on the 
>> particular platform)
>> 4. JavaFX encapsulates this "new" image with Object
> Technically, snapshot does its own (hardware accelerated) rendering 
> into a separate GPU texture/FBO and does a read pixels on that 
> texture.  It then encapsulates those pixels into an Image object.
> This snapshot operation need only be done once if the underlying node 
> tree is static.
>> 5. We draw the "new" image with effects to the OGL context with the 
>> same node-tree API as we did before on top of the first node-tree.
> Correct.  Note that when you render that Image object its pixels are 
> cached in vram and that vram copy is reused from frame to frame.  So, 
> if you reuse the one snapshot then there was only one trip from vram 
> to memory and back to vram on the first frame you did this and then 
> everything should be done in vram for subsequent frames reusing the 
> same snapshot.
>> That works, but there is too much CPU and memcpy involved for my 
>> believe. Furthermore ReadPixels takes forever in
>> an OpenGL perspective.
> If the underlying tree is static, though, that readpixels operation 
> only happens once at the start of the operation, but you are correct 
> that it would be much better if it didn't need to happen at all.
>> Since this is just one very simple effect, it is actually not good to 
>> spend more than approx. 20% CPU (i7) load on it.
>> I expect 0% (not noticable) for this blurry effect.
> That would be ideal.  We may need new API to get there, but there are 
> also options to consider that may get us there in the short term.  In 
> particular, cached nodes - which are already present in the API - may 
> get us closer to that goal.
>> The ideal implementation from my perspective would be:
>> 1. the application constructs a JavaFX node tree.
>> 2. the node tree is rendered mostly in hardware (shader effects and 
>> so on on top) to a virtual framebuffer in the GPU space
>> 3. The virtual framebuffer is drawn by a simple drawVert-call for the 
>> background
>> 4. The virtual framebuffer is drawn once again shaped and shaded 
>> (blurry filter) by another drawVert-call
> If you set the cache hint to true on the underlying tree then this may 
> be approximately what happens in our current implementation.  The part 
> I would still need to investigate would be how well the effects 
> machinery (called Decora) can reuse the cached version of the nodes.  
> If it doesn't attempt to reuse the cached version of the nodes then it 
> might end up re-rendering the tree.

This would an interesting experiment, and would let us know how easily 
applications could do what they need with the existing API. If there are 
limitations or performance bottlenecks we can look into fixing them.

>> I have to correct my assumption that JavaFX renders most of its stuff 
>> in software a bit. The wording "rendering" was not correct.
>> This feeling comes probaly from the massive use of Snapshot. I have 
>> not understood the entire core of Prism until now.
>> In some implementation of snapshot it seems to do the rendering 
>> entirely in software. That might only the fallback, but the
>> ReadPixels-Stuff is still valid.
> To be clear, snapshot was mainly created for its namesake - to produce 
> static copies of the scene data to be saved as images for putting into 
> documentation.  Alternate uses that were considered were for 
> generating thumbnails of scenes for an application that could 
> open/close various scene graph panes.  One wouldn't use the Windows 
> "print screen" API in a performance intensive part of one's 
> application either.  It is great that it can be used for this 
> particular result, but it was not designed to be performant in that 
> respect.  Note that the primary snapshot API is the asynchronous 
> version with the callback to deliver the data.  The non-asynchronous 
> version is mostly just a helper around that, but it stalls the 
> rendering pipeline to complete its work.  None of that was designed 
> for insertion into a running animation technique.


>> Tiny-Offtopic: I already meantioned on this list that JFXPanel 
>> (Swing) is way below it's expectations. The main reason there is also 
>> the use
>> of the snapshot function instead of letting render JavaFX in it's own 
>> heavy weight window. In our small test case a simple List
>> displayed via JFXPanel was not even to render more than 1 fps and it 
>> slowed down the entire Swing-UI. Since I know jogle and it's canvas
>> implementation, would it be good to have a similar heavy weight JFX 
>> canvas in Swing? This would let JFX use it's full hardware rendered 
>> power.
> It doesn't use snapshot per-se, but it does do something similar. 
> Sharing contexts and GPU resources with AWT/Java2D is on a wish list, 
> but architecturally we aren't there.  Kevin would know more about our 
> challenges on that front than me...

I haven't looked recently at what it would take to share resources 
between Java2D and AWT, but that seems separate from the question of 
whether we could use a heavy-weight JFXCanvas as opposed to the current 
lightweight JFXPanel. We haven't considered that since there are 
limitations with heavyweight components. I am not aware of any 
performance problems that are as serious as "simple List displayed via 
JFXPanel was not even to render more than 1 fps". Do you have a test 
case? I do note that we did have a bug on Mac retina displays that was 
recently fixed which killed performance. Was that what you were running 
your 1fps program on?

-- Kevin

>             ...jim

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