private unsafe MeshVertex CreateVertex(CyCoord coords, float dTheta)
{
Vector3 *pos = stackalloc Vector3[3];
CyCoord tcoord = coords;
for (int i = 0; i < 3; i++)
{
//compute 3 positions for computing the derivate respect theta
pos[i] = tcoord.ToCartesian();
tcoord.Theta += dTheta;
}
var tangent = Vector3.Normalize(Numerics.DerivateForward1(pos, dTheta));
var normal = Vector3.Normalize(Vector3.Cross(tangent, new Vector3(0, -1, 0)));
var texCoord = new Vector2(coords.Theta / Numerics.TwoPI, 0.5f - coords.Y / height);
return(new MeshVertex(pos[0], normal, tangent, texCoord));
}