How do I interpolate normals on Catmull-Clark Subdivision Surfaces

Question

I'm using CCSS to generate smooth surfaces.

I've been using the regular subdivision rules to interpolate the surface/vertex normal, but I think this may be wrong.

Are there different stencils to interpolate normals?

Answer 1

The "normals" from the control mesh are not really normals to begin with. They're just made-up vectors at each vertex, and not something you want to interpolate.

Instead, use the derivative stencils, which yield tangent vectors in two directions. Once you have your tangent vectors, cross them to get a normal. The derivative stencils are:

1   4  1
0  (0) 0
-1 -4 -1

and

-1  0  1
-4 (0) 4
-1  0  1