Hi all

this is my first post on this forum.

First of all i want to say you that i've searched many posts on this forum about this specific argument, without success, so i write another one....

Im a beginner.

I want use GPU geometry clipmaps algorithm to visualize virtual inifinte terrains. 

I already tried to use vertex texture fetch with a single sampler2D with success.

Readed many papers about the argument and all speak about the fact that EVERY level of a geometry clipmap, has its own texture. What means this exactly? i have to 

upload on graphic card a sampler2DArray?

With a single sampler2D is conceptually simple. Creating a vbo and ibo on cpu (the vbo contains only the positions on X-Z plane, not the heights)

and upload on GPU the texture containing the elevations. In vertex shader i sample, for every vertex, the relative height to te uv coordinate.

But i can't imagine how can i reproduce various 2d footprint for every level of the clipmap. The only way i can imagine is follow:

Upload the finer texture on GPU (entire heightmap). Create on CPU, and for each level of clipmap, the 2D footprints of entire clipmap.

So in CPU i create all clipmap levels in terms of X-Z plane. In vertex shader sampling these values is simple using vertex texture fetch.

So, how can i to sample a sampler2DArray in vertex shader, instead of upload a sampler2D of entire clipmap?

Sorry for my VERY bad english, i hope i have been clear.

Typically you store the clipmap levels as mipmaps in a single texture. Then you select the correct mipmap during rendering via an LoD parameter to the texture sampling function.

Ok. But say i have a complete  2d footprints of a clipmap (12 blocks mesh, two L-shaped meshes and the cross mesh), how can i, in vertex shader, reproduce the various scaled versions (1 for each level)? The only solution i can think is produce on CPU all scaled version of 2D vertex positions, and it means that i produce (if say L the number of clipmap levels) L versions of the 2D footprints and send them on GPU. 

So there is a difference between "Geometry Clipmaps" and "Texture Clipmaps".

Geometry Clipmaps are described here: http://hhoppe.com/geomclipmap.pdf

The algorithm is a continous LOD for the mesh

Texture Clipmaps are described here: http://developer.download.nvidia.com/SDK/10/direct3d/Source/Clipmaps/doc/Clipmaps.pdf

The algorithm handles the visualization of very large textures by using a stack of LOD textures as opposed to the usual pyramid.

I used Geometry Clipmaps in the early days (before switching over to a quad tree based algorithm), and I used a toroidal addressing method for the texture (See the Texture Clipmaps paper for an explanation)

To answer your question: You can scale and offset your parts in the vertex shader using data from a constant buffer eg.

pos_base = pos_input_vs + meshOffset;
tex_coord = pos_base.xy
pos_world = pos_base * LODscale + LODoffset

Where meshOffset is to place the L-shaped part at the right location and LODscale and LODoffset is to get it into the right size and position in the world.

Henning