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Rendering large amount of transparent objects


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#1 Plerion   Members   -  Reputation: 363

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Posted 09 May 2013 - 09:02 AM

Hello gamedev

 

When rendering a lot of transparent objects ive come across two problems (one is actually caused by the other and vice versa).

 

1. If i render all instances of the same object using hardware instancing i have problems with alpha blending and alpha testing, but FPS is great (158 frames per second for the same part as in point 2)

 

2. If i render opaque models still with instancing and transparent ones instance by instance according to their depth in the scene graph everything looks perfect, but the FPS drops to 30 frames per second.

 

Here is an image showing the problem: The trees are transparent but all the same model. With instancing for everything ~160 FPS, without for transparent objects 30

518bb70b7c7b7_models.jpg

 

 

I used the profiler in visual studio as i expected some issues in my scene graph and/or general rendering process but what i have seen there showed me something entirely different. Rather than trying to describe ill just post two pictures:

518bb9ba587db_Calltree.jpg

 

518bb9d621b79_HotPath.jpg

 

For me this means that the actual rendering is completely responsible for the loss of FPS. But how can i change this? In order to get results that are acceptable i have to remove instancing for transparent models but in order to have playable FPS i need to add instancing.

 

Any help is appreciated :)

 

Thanks a lot in advance,

Plerion



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#2 Juliean   GDNet+   -  Reputation: 2350

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Posted 09 May 2013 - 10:34 AM

You simply need to set up the instance buffer in the same depth-sorted order than you would render the objects without instancing. Said, farest away goes in first, nearest last. Instanced meshes are quaranted to being drawn in the order of the instance buffer. This should solve your alpha blending/testing problems and you should be able to use instancing.

 

PS: I hope you this if for fun/learning only, I quess blizzard wouldn't be too happy about you using their graphics ;)


Edited by Juliean, 09 May 2013 - 10:35 AM.


#3 Plerion   Members   -  Reputation: 363

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Posted 09 May 2013 - 10:39 AM

Hello Juliean

 

I have thought of that but when ive drawn the following situation i was thinking it will still give wrong results: Imagine a transparent plane A and another transparent plane B:

A
B
A
B

 

^

|

Camera

 

Now i first render the instances of A (orderd by depth) and the the same for B. Wouldnt the second A (closer to the camera) blend with the other A and not at all with the instance of B further away?

 

Greetings

Plerion



#4 Juliean   GDNet+   -  Reputation: 2350

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Posted 09 May 2013 - 11:38 AM

Ah, I see, that makes it a bit more complicated. From what I get you basically need an order-independant transparecy rendering technique.

 

http://publications.lib.chalmers.se/records/fulltext/173349/local_173349.pdf seems to be about that, didn't read it in-depth though, but you might give it a look.



#5 C0lumbo   Crossbones+   -  Reputation: 2155

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Posted 09 May 2013 - 11:39 AM

Hello Juliean

 

I have thought of that but when ive drawn the following situation i was thinking it will still give wrong results: Imagine a transparent plane A and another transparent plane B:

A
B
A
B

 

^

|

Camera

 

Now i first render the instances of A (orderd by depth) and the the same for B. Wouldnt the second A (closer to the camera) blend with the other A and not at all with the instance of B further away?

 

Greetings

Plerion

 

You still might be able to use a limited amount of hardware instancing in this case. For instance, if 4 'A's appeared in a row, you could render the 4 of them with a single draw call.

 

The other thing to look at is techniques so that you don't need to sort your objects back-to-front. In your current example, your trees are mostly on/off alpha. You don't go into details about what artifacts you're getting when you don't sort. If you have great chunks of objects missing, then you can easily improve on that with an appropriate discard/clip added to your pixel shader.

 

If you're worried about more subtle feathering artifacts on the edges of your leaves, then they can be trickier to deal with. It helps to render the opaque stuff first and do any coarse back-to-front sorting you can manage. You might also improve the look by fiddling with the alpha threshold you use for discard/clip but the whole thing is an awkward balancing act. This Wolfire blog post suggests an approach of drawing everything twice : http://blog.wolfire.com/2009/02/rendering-plants-with-smooth-edges/

 

Another approach is looking at alpha-to-coverage if you're multisampling.



#6 Plerion   Members   -  Reputation: 363

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Posted 10 May 2013 - 02:59 AM

Hello

Thanks for your tips. Ive selected two algorithm and will implement them now, hopefully it will help smile.png

Greetings
Plerion




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