function srf = nrb4surf(p11,p12,p21,p22)
% 
% NRB4SURF: Constructs a NURBS bilinear surface.
% 
% Calling Sequence:
% 
%   srf = nrb4surf(p11,p12,p21,p22)
% 
% INPUT:
% 
%   p11		: Cartesian coordinate of the lhs bottom corner point.
% 
%   p12		: Cartesian coordinate of the rhs bottom corner point.
% 
%   p21		: Cartesian coordinate of the lhs top corner point.
%  
%   p22		: Cartesian coordinate of the rhs top corner point.
%
% OUTPUT:
% 
%   srf		: NURBS bilinear surface, see nrbmak. 
% 
% Description:
% 
%   Constructs a bilinear surface defined by four coordinates.
% 
%   The position of the corner points
% 
%          ^ V direction
%          |
%          ----------------
%          |p21        p22|
%          |              |
%          |    SRF       |
%          |              |
%          |p11        p12|
%          -------------------> U direction
% 
%
%    Copyright (C) 2000 Mark Spink
%
%    This program is free software: you can redistribute it and/or modify
%    it under the terms of the GNU General Public License as published by
%    the Free Software Foundation, either version 2 of the License, or
%    (at your option) any later version.

%    This program is distributed in the hope that it will be useful,
%    but WITHOUT ANY WARRANTY; without even the implied warranty of
%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%    GNU General Public License for more details.
%
%    You should have received a copy of the GNU General Public License
%    along with this program.  If not, see <http://www.gnu.org/licenses/>.

if nargin ~= 4
  error('Four corner points must be defined'); 
end

coefs = [zeros(3,2,2); ones(1,2,2)];
coefs(1:length(p11),1,1) = p11(:);    
coefs(1:length(p12),1,2) = p12(:);
coefs(1:length(p21),2,1) = p21(:);
coefs(1:length(p22),2,2) = p22(:);
             
knots  = {[0 0 1 1] [0 0 1 1]}; 
srf = nrbmak(coefs, knots);

end

%!demo
%! srf = nrb4surf([0.0 0.0 0.5],[1.0 0.0 -0.5],[0.0 1.0 -0.5],[1.0 1.0 0.5]);
%! nrbplot(srf,[10,10]);
%! title('Construction of a bilinear surface.');
%! hold off