[Gmsh] "Ruled" surface mathematical definition

[Philippe Grenard]

Hi all,

I would like to understant how the "ruled surfaces" in gmsh are mathematically 
defined.
I mean, let's say I have 4 lines (curved, spline or anything)
These can be discretized by 4 sets of points  P(i,j) :
P(i=0            , j=1,..,,N-1)
P(i=1,...,M-1 , j=0)
P(i=M           , j=1,..,,N-1)
P(i=1,...,M-1 , j=N)

How are the internal points P(i,j) of the surface defined ?

for example :
Point(1) = {0, 0, 0, 0.04};
Point(2) = {0, 1, 0, 0.04};
Point(3) = {1, 0, 0, 0.04};
Point(4) = {1, 1, 0, 0.04};
Point(5) = {1, 1, 1, 0.04};
Line(1) = {1, 2};
Line(2) = {1, 3};
Circle(3) = {5, 4, 3};
Circle(4) = {5, 4, 2};
Line Loop(5) = {2, -3, 4, -1};
Ruled Surface(6) = {5};

How can I define this surface (as a set of points for example) ?

Thank you for your help,

Philippe