How do we apply different textures to different faces generated via geometry shader?

Question

I am trying to render a cube by starting of with one vertex into the shader, then geometry shader to expand into a cube, and then apply a different texture to each side. The part I am facing the issue with applying unique textures to each side.

So I am currently doing this by creating it via quads pushing the vertices, and applying the textures one by one. In fact, I drawIndexed() a set of vertices, then switch over to a different textures every 6 vertices. But pushing in so many vertices is expensive for higher FPS. So, I looked in to Geometry shader and it seems like it did the job. After studying it, I understand how to develop the vertices in the geometry shader. However, I don't know how to apply a different texture say every 6 vertices - since we are in the pipeline already.

Answer 1

The bound textures in a single draw call cannot be changed. This means that your geometry shader cannot select arbitrarily between different textures.

The solution is to use either texture atlasing or array textures. In either case, for each cube face you generate both new positions and new texture coordinates.

The C++ code must still bind the correct atlas or array that covers the needs of all faces of a cube. Games like Minecraft need only a single atlas for all cubes; they could also have used an array if they targetted newer versions of GL.

In either case, your geometry shader needs to sample some information from the vertex attributes that indicates which 6 slices of the atlas/array are used for each face. It then uses that information to set the proper texture coordinates.