How to do bilinear interpolation of normals over a quad?

Question

I\'m working on a Minecraft-like engine as a hobby project to see how far the concept of voxel terrains can be pushed on modern hardware and OpenGL >= 3. So, all my geometry con

Answer 1

Well... as you're using Bent normals technique, the best way to increase result is to pre-tessellate mesh and re-compute with mesh with higher tessellation.

Another way would be some tricks within pixel shader... one possible way - you can actually interpolate texture on your own (and not use built-in interpolator) in pixel shader, which could help you a lot. And you're not limited just to bilinear interpolation, you could do better, F.e. bicubic interpolation ;)

Answer 2

As you clearly understands, the triangle interpolation that GL will do is not what you want. So the normal data can't be coming directly from the vertex data.

I'm afraid the solutions you're envisioning are about the best you can achieve. And no matter what you pick, you'll need to pass down [0..1] coefficients from the vertex to the shader (including 2x2 textures. You need them for texture coordinates).

There are some tricks you can do to somewhat simplify the process, though.

• Using the vertex ID can help you out with finding which vertex "corner" to pass from vertex to fragment shader (our [0..1] values). A simple bit test on the lowest 2 bits can let you know which corner to pass down, without actual vertex data input. If packing texture data, you still need to pass an identifier inside the texture, so this may be moot.
• if you use 2x2 textures to allow the interpolation, there are (were?) some gotchas. Some texture interpolators don't necessarily give a high precision interpolation if the source is in a low precision to begin with. This may require you to change the texture data type to something of higher precision to avoid banding artifacts.