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Relfos

OpenGL How to optimize scene rendering

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Relfos    349
I was building an map editor for our RPG game, but as the features and complexity of the maps raise, some more optimizations are needed. As you can see in the screenshot, the polycount of the scene is rather low for today standarts. Also, basic boundingbox-frustum test are used to cull invisible objects out of the rendering. The "Chunks" is the number of mesh chunks passed to the renderer, the "Batches" the number of actual glDrawElements calls for 3d geometry. The "Spr.Batches" is the number of glDrawElements calls for 2d geometry. About 200 sprites are passed in, but they're sorted into batches and only 6 calls are done, so I think this dont need more optimizations for now. Ignore the "Mirrors" and "Lights" for now, as they're not used in this scene. The "Shaders" is the number of shader switches per frame, and the "Transfer" the ammount of texture memory swapped around per second. About the current optimizations. The renderer sort objects by alpha, then by texture, then by distance. (Shaders are not taken into the sort function, should I include them?) All static meshes are rendered using VBOs, however the animated ones are updated in the CPU (both skeletal, morphs and keyframe animation). A cache is used per instance, and only modified when needed (eg: The updates are only done at 24fps). I need to store the modified geometry myself since then its used for generating both mesh outlines and shadow volumes. About the shadow volume itself, its done in the CPU, and even if I disable it in this scene, only gain about 5 or 6 more fps. All textures and shaders are cached, so as long as sort the objects correctly, minimal changes are done. Also, every shader uniform is cached, to minimize changes. The sky itself is a dynamic skydome, however its also rendered fast in one GL call. The whole scene is rendered using a modified parallax mapping algoritm that uses 3 texture units but its very simple and lightweight. Finally, theres a post process shader that is rendered as a fullscreen quad, however its quite fast, disabling it doesnt change the fps it seems. So I need tips to optimize this, please. How to sort objects for minimizing gl status changes? Its more expensive swapping a shader or a texture? Its there any openGL tools to check where the rendering bottlenecks are? The terrain geometry is completely unoptimized, even a completely flat surface will have lots of polys generated. Should I try to implement some form of geo-mipmapping or even LOD for the trees and stuff?

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swiftcoder    18426
Have you profiled to see where the bottleneck is? I love the debug info you have in your HUD, but that isn't really enough to go on. In particular, are you GPU bound or CPU bound (I would assume the former)?

First off, I am pretty sure that switching shaders ought to be more expensive than switching textures - most engines sort by material rather than texture.

Secondly, by my estimation you are only rendering about 30 triangles per batch, which is very low for modern cards. Can you increase this at all?

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Relfos    349
No, I dont have much experience profiling code, and I'm using Delphi so I really dont have any idea how to do it.
But probably CPU bound, as the scene geometry is far from complex for my gfx card.
Already changed the sorting to include shaders and the fps increased a little.

"Secondly, by my estimation you are only rendering about 30 triangles per batch, which is very low for modern cards. Can you increase this at all?"

Hmmm, as you can, most of the meshes in the scene are very low poly trees.
How can I batch more than one mesh per call, since every mesh has it own transformation matrix?

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Quote:
Original post by Relfos
How can I batch more than one mesh per call, since every mesh has it own transformation matrix?
Either by using the instancing extension (only available on the newest nVidia cards) or by pseudo-instancing. Interestingly, pseudo-instancing seems to be no slower than "real" instancing (at least not measurable).

For pseudo-instancing, you pass your transform matrix (or something else) as color or texture coordinate using the glTexCoord or glColor functions (or glVertexAttrib ). Note that you do not have to add something extra to your vertex buffer data, but you use these functions to set the state once before rendering each object. It is the same as you would do in immediate mode (although it is not immediate mode!). This is very little overhead and effectively lets you draw hundreds of trees in one batch.

In your vertex shader, you figure out what to make of that data. It is pretty much up to you what you want to do. For example, if you want to be able to scale your trees and place them at different locations, but you don't care about orientation, that will fit neatly into one attribute. Construct a matrix from it, and multiply.

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deathkrush    350
Quote:
Original post by Relfos

About the shadow volume itself, its done in the CPU, and even if I disable it in this scene, only gain about 5 or 6 more fps.



Only gain 5 or more fps? That's a lot of gain for this particular scene.

12 FPS = 83 ms.
12 + 5 = 17 FPS = 58 ms.
Speed gain = (83 - 58) / 83 = 30%

If the framerate with shadows was 30 FPS, it would probably be around 44 FPS with shadows turned off. Please stop using FPS to measure framerate and use milliseconds per frame instead. Speed gains are more obvious that way.

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Shinkage    595
Yeah, it's really hard to say where the slowdown is without a little profiling. I believe the newer high end versions of Visual C++ come with a profiler, although I moved to Linux before that was the case so I don't know too much about it. I don't know any free Windows profilers off the top of my head, but since you're doing OpenGL you could always install some flavor of Linux and use Valgrind, whish is about as powerful as they come ;) Seriously though...

My first observation though, is that 14k polygons is a LOT for a scene of the complexity I'm seeing in that screenshot. Is that how many are being passed to the renderer, or are a lot of those being culled out? I wasn't too clear on how you described that.

One thing I would try is simply turning off ALL of the special effects in the scene, then turning them on again one by one to see where the biggest performance hits come in. That is, first start by rendering it as a completely static scene with no shadows, no fog, and only a simple passthrough shader that doesn't do anything fancy. Then activate all those features individually, and test the speed that way (i.e. test with ONLY shadows, ONLY fog, ONLY animation, ONLY advanced shaders). With those results you should be in a better position to try out specific optimizations or come back with more detailed information. Also, I don't know what video card you have, but my wild guess is the parallax mapping you're doing would be very slow on anything that isn't fairly recent.

The best suggestion I can think of off the top of my head would be to implement some kind of space partitioning structure (assuming you aren't using one) for the entire scene. A basic quadtree for example would be a simple one to implement, and would probably give a nice boost to both the culling and depth sorting.

Also, an instancing technique, as mentioned earlier, might help a little, although with that particular scene it doesn't look like its effect would be that great (just on the trees really, and there aren't all that many of them).

I don't know if you're doing any kind of dynamic level-of-detail work on your terrain, but that might be something to look into to reduce the polygon count being passed to the renderer. I would definitely save this for last though, because dynamic LoD on terrain is pretty complex just by itself.

And finally, is that compiled in debug or release mode?

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