A NURBS mesh is a series of NURBS patches. The NURBS mesh is described in a .nrb file. following this format.
Look at the definition of a .nrb file:
{ NURBS
{ patch
orderx 4;
ordery 4;
dimx 4;
dimy 4;
{ 0; 1; 2; 3; 4; 5; 6; 7; }
{ 0; 1; 2; 3; 4; 5; 6; 7; }
(1.40000, 0.00000, 2.40000),1;
(1.40000, -0.78400, 2.40000),1;
(0.78400, -1.40000, 2.40000),1;
(0.00000, -1.40000, 2.40000),1;
(1.33750, 0.00000, 2.53125),1;
(1.33750, -0.74900, 2.53125),1;
(0.74900, -1.33750, 2.53125),1;
(0.00000, -1.33750, 2.53125),1;
(1.43750, 0.00000, 2.53125),1;
(1.43750, -0.80500, 2.53125),1;
(0.80500, -1.43750, 2.53125),1;
(0.00000, -1.43750, 2.53125),1;
(1.50000, 0.00000, 2.40000),1;
(1.50000, -0.84000, 2.40000),1;
(0.84000, -1.50000, 2.40000),1;
(0.00000, -1.50000, 2.40000),1;
}
}
.
This file contains only a patch, but you can have .nrb files with more patches.
orderx and ordery are the order of the surface for x and y parameters.
dimx and dimy are the dimension of the array of control points.
The two sections without names are the knot section for x and y parameter; the follows a the list of control points, in the form:
( x , y , z ) , w;
where w is the weight.
If you have .nrb file you can add it to a .rt file in the same way that a .brp file.