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mikeman

OpenGL Grass rendering

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Hello. I've come to a point where I want to implement grass for my 256x256 terrain(OpenGL). I want it to be covered by grass. I thought it was a good idea to divide the terrain in cells, say 16x16, allocate a VBO for each cell containing the grass blades geometry, and render only the cells that are visible. However, 1x1 is in fact a relatively large area in the game, so the density of the grass looks good if there are rendered,say, 300 or grass 'quads' in it. So, we have: 256 x 256 x 300quads x 2trianles x 3coords x 4bytes= ~450MB ! Clearly, requiring 450 mb just for the grass geometry is unacceptable. However, if I don't batch the geometry that way I lose performance. So what am I missing here? What is the correct way to have both batching and acceptable memory requirements?

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use instancing...

define maybe 3 different patches... that u place randomly and in different directions to make it look random

EDIT:

u can get some ideas from here, http://www.irisa.fr/bunraku/GENS/kboulang/publications/grassSiggraph2006ppt.pdf

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Thanks for the reply. The problem is:

1)OpenGL supports instancing only for GeForce 8 hardware.

2)Yes, I've thought about allocating only a few patches and use them repetitively to cover the whole terrain. The thing is, the height of which the grass blades are rendered has different distribution for each cell. The obvious solution is to store the heightmap in a texture and do a lookup in the vertex shader, correctly positioning the blades, but again texture lookups in vs are not supported by older hardware.

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What I did for a project using older hardware was to store the index of the item (rocks in my case) in the chunk of data (i.e. I have 8x8 area of rocks, uniformly spaced, all in one buffer - each has an index in the vertex, think I used BLENDINDEX semantic or whatever it is). Each time I passed the material based on that vertex data, I also passed an array with 64 float4 elements. Here I can do all sorts of position perturbations, including taking into account the height of the ground underneath.

Works well and was very fast. If you use smaller chunk sizes, you can probably fit it down to VS 1.1 if you wanted to.

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Quote:
Original post by mikeman
Thanks for the reply. The problem is:

1)OpenGL supports instancing only for GeForce 8 hardware.

Nope. NVidia Developer Instancing - whereas instancing on Geforce 8 might be faster.

GPU Gems 2 has an interesting article about landscape rendering (very first article). The second good paper was already mentioned above.

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Quote:
Original post by Enrico
Quote:
Original post by mikeman
Thanks for the reply. The problem is:

1)OpenGL supports instancing only for GeForce 8 hardware.

Nope. NVidia Developer Instancing - whereas instancing on Geforce 8 might be faster.

GPU Gems 2 has an interesting article about landscape rendering (very first article). The second good paper was already mentioned above.


Yeah, that's pseudo-instancing that I'm already aware of and considering it. Still, it requires one draw call for each instance. True instancing, the way D3D does it, does require GeForce8; it says so in the extension spec.

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Quote:
Original post by mikeman
Yeah, that's pseudo-instancing that I'm already aware of and considering it. Still, it requires one draw call for each instance. True instancing, the way D3D does it, does require GeForce8; it says so in the extension spec.

So then what is wrong with the GPU Gems 2 article or the paper mentioned above? Both don't need any instancing...

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Yeah, that's pseudo-instancing that I'm already aware of and considering it. Still, it requires one draw call for each instance. True instancing, the way D3D does it, does require GeForce8; it says so in the extension spec.


It doesn't require one draw call per instance...otherwise there is no point.

I believe it uses shader instancing (or constant instancing)..which basically means you have a big vertex buffer full of polygons, and each vertex has an index stored in a texcoord. You can essentially render as many instances per-call as you have constants available.

However, you may not even need instancing in GL for this, as GL doesnt have the kernel-mode switch for each draw call like Direct3D.

----------------

I really think vertex texture are huge win for massive fields of grass, but otehrwise you can use meshes with LOD and or billboards.

Each mesh or instance has to "know" what height it is to be placed at, so unless you use a vertex texture, you needs a way to read the heightmap or terrain mesh y value at a given point and send that to the shader...but your game engine has to have this ability anyway right? otherwise how is collision with terrain done?

----------------

As far as i can see there is no one right way to do grass...Oblivion used randomly placed instanced meshes, as does Crysis I believe...A game like Two Worlds used simple billboards.

My own method uses large patches of sheets with varying grass texture and is placed at the correct height by a vertex texture fetch..this allows me to do a very large field of grass.

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a good way is to create grass mesh brushes.
instead of rendering single grass you can render a brush mesh that holds more
grasses. have different kind of grass brushes so that you have different style of the grass brushes. create a table with informations about position of the grasses ofcourse bind them to your patch culling informations of the terrain to save culling calculations. so the only thing you need to hold in memory is a world matrix of the grass brush. since grass is static you can at startup calculate all the world matrices for the brushes that are on the terrain.
another speed up is to have all the brush meshes in one vertex buffer and the same for the indices. so you dont need to switch vertex buffers. ofcourse you can pack it into the terrain vertex buffer you have.

i have done this in directx but should not be a problem to do the same with opengl vertex buffers.

how big you do the grass brushes depends on your terrain etc... just test which brush size fit best to you terrain.

hope that helps your. ( sorry for bad english )

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