I was going through this ufunc API UF_MODL_ask_face_props. I am having hard time understanding the output of this API. I am not sure about the First derivative in U, First derivative in V, Second derivative in U, Second derivative in V. What is the physical meaning of these derivatives ? What exactly it returns when i query a face ? Kindly help me to understand this output.
In NX, as in most CAD software, each face (or the underlying surface, really) has a parametric equation, S(u,v). The derivatives mentioned are the partial derivatives of the function S with respect to u and v respectively. The concept of "partial derivative" is standard mathematics, so you can look this up anywhere. The only physical significance is that the surface normal is usually the vector cross product of the (first) partial derivatives. I say "usually" because occasionally the cross product is zero, in which case things get a bit more complicated.