UF_MODL_ask_face_parm函数UF_MODL_ask_face_props函数返回值 代表的意思

取个什么名好

int UF_MODL_ask_face_parm
(

tag_t face_id,
double ref_pnt [ 3 ] ,
double parm [ 2 ] ,
double face_pnt [ 3 ]

)

[url=mkMSITStore:C:\Users\lis6ar\Desktop\net_ref.chm::/ugopen_doc/uf_defs/uf_defs.html#tag_t]tag_t[/url]	face_id	Input	Face identifier.
double	ref_pnt [ 3 ]	Input	Reference point in absolute space (ref_pnt[3]).
double	parm [ 2 ]	Output	Parameter (u,v) on face (parm[2]).
double	face_pnt [ 3 ]	Output	Point on the surface of the given face (face_pnt[3])

请问Parameter (u,v) 代表的是什么意思？当face_id代表一个球面的时候，返回(3.1415..,0)


int UF_MODL_ask_face_props
(
tag_t face_id,
double param [ 2 ] ,
double point [ 3 ] ,
double u1 [ 3 ] ,
double v1 [ 3 ] ,
double u2 [ 3 ] ,
double v2 [ 3 ] ,
double unit_norm [ 3 ] ,
double radii [ 2 ]

)

[url=mkMSITStore:C:\Users\lis6ar\Desktop\net_ref.chm::/ugopen_doc/uf_defs/uf_defs.html#tag_t]tag_t[/url]	face_id	Input	Face identifier.
double	param [ 2 ]	Input	Parameter (u,v) on face (param[2]).
double	point [ 3 ]	Output	Point at parameter (point[3]).
double	u1 [ 3 ]	Output	First derivative in U (u1[3]).
double	v1 [ 3 ]	Output	First derivative in V (v1[3]).
double	u2 [ 3 ]	Output	Second derivative in U (u2[3]).
double	v2 [ 3 ]	Output	Second derivative in V (v2[3]).
double	unit_norm [ 3 ]	Output	Unit face normal (unit_norm[3]).
double	radii [ 2 ]	Output	Principal radii of curvature (radii[2]).

请问，u1，v1，u2，v2，Unit face normal代表的是什么意思？
谢谢。