Hi there,

I'm looking for a way to apply a heightmap to a terrain.
I want to do this GPU-side because doing it on the CPU is too slow.

Has anyone got an idea for this?

regards, Alex

I don't think that this is possible because tex2D or similar commands are (with a few exceptions) not supported in vertex shaders.

Thanks for the answer!

storing the information in an array could work...
but I'm not sure if shader model 3.0 supports enough indices for doing this.

yea, it doesn't support enough...

Well, actually the intrinsic function tex2D and tex2Dlod can be used in a vertex shader to sample a 2D texture, but you require at least Shader Model 3 for it, plus, the graphics card has to support that. For example the function is only supported on NVidia cards from GeForce 6/7 Series and higher, and NV4x-based Quadro FX; for ATI cards I don't have any information about that. Besides, working with matrices on the CPU is not a big deal. I don't know what you are doing and how big the heightmap is, but maybe you should consider to write a multithreaded routine that transfers the contents of a matrix into the mesh on the CPU prior rendering.

another question, is it possible to modify vertex positions via shaders
I mean constantly modifying, not per frame modification.

What do you mean exactly? What do you want to do?

You can set them to a value read from a heightmap or from wherever, but this won´t effect the meshs representation within the engine, just the output. And you can´t set the vertex positions in one frame and add to those positions in another frame, what you can do is just setting them to whatever value you want within the vertex shader, which is processing each vertex every frame the mesh is rendered.
If you want for example an "endless terrain" which is always moving with the player and reads the vertices height values from a heightmap, you can do this very fast in a shader, while doing this in your script would be really slow.

A realtime terrain editor. My thought was it to have a heightmap I can "draw" on.
Within the shader I want to set vertex.y based on the correspondending pixel of the heightmap.



That's what I wanted to do, but VS doesn't support texture-reading functions like tex2D / 3D / whatever.
I also thought about storing the information in an array but SM 3.0 doesn't support enough indices for this method...

Why don't you manipulate the vertices close to the mouse/ brush on the CPU? This should be really fast in your case. If you want you can still calculate a heightmap from your vertices for whatever reason.