Computing normals on a cube : vertices or shaders?

Question

I have a quite theorical question about normals in OpenGL. I could not find a clear explanation anywhere (my knowledge in this subject is not spot on).

I would like to put textures on a Cube, one per face, and the most obvious way to do this is playing with normals.

But including normals in vertices leads to not sharing these vertices :

• On a Cube with shared vertices, we would have 8vertices.
• On a Cube with non-shared vertices, we would have 8*3 = 32vertices.

I've read somewhere that shaders can be used to determine normals and apply textures, but I don't really know anything about shaders and was wondering what was the best way to do this work (apply textures on the cube's faces).

On top of that, I would like these vertices to not share the coordinates of the texture applying to them (for example, the vertex on the bottom-left in front of us would have a (0,0) UV coordinates for the texture on the FRONT face of the cube, a (0,1) UV coordinate for the texture on the LEFT face of the cube, and a (1,1) UV coordinate for the texture on the BOTTOM face of the cube). Can shaders achieve this work when patching the texture ?

In conclusion, what is the best way to apply a non-shared texture & non-shared UV coordinate to a vertice : with non-shared vertices (a lot more vertices), or with a shader (more work for the GPU, i guess) ?

Answer 1

I would say this depends on your use case. Consider the following:

• How many vertices do you have to render? If only few, saving mesh storage space is not very important.
• How often are vertices transferred from CPU to GPU? If it is necessary every frame, saving vertices might speed up rendering time.
• Are most of you edges non-smooth? If most edges should appear smooth, than you could only break the mesh for the few cases where it is necessary.

For all my hobby projects I went with non-shared vertices, as it is often easier to handle. I would care about performance only if it shows to be a bottle neck in your program.