If I know the coordinates of (say) all the vertexes of a face F and all the vertexes of all adjacent faces, please what is a quick way to find the coordinates of another point that is on a curved surface drawn through all those vertexes? Say: I rotate the coodinate axes so that face F is flat in the XY plane; if I need a point {x,y,z} which is on a surface drawn through all those vertexes, and I know x and y, how to quickly find z?