I’m trying to calculate smooth normals for a cone. In looking around for code samples and explanations, I consistently come across directions for face normals. I’ve posted a couple pictures below of what I’m doing. The first — which basically just normalizes the vertex position — gives me decently smooth shading, but the edges are “missing” and the bottom face isn’t solid. The second has edges, but the shading is flat (face normals) and my light isn’t reflecting off of them correctly.
The cone is built out of GL_TRIANGLES.
Click the images for larger versions.
(source: bantherewind.com)
(source: bantherewind.com)
At any point on the surface of a cone except the apex, there are two obvious kinds of tangent vectors: one tangent to the cross-sectional circle, or one up the slope. If you express the surface as a parametric equation with two parameters, you can get these tangent vectors as the two partial derivatives. Take the cross product of the tangents, and you get a normal vector. The order of the product determines whether the normal points inward or outward. Of course, the bottom face must be handled separately.