polylinePoint

POLYLINEPOINT Extract a point from a polyline.

   POINT = polylinePoint(POLYLINE, POS)
   POLYLINE is a N*2 array containing coordinate of polyline vertices
   POS is comprised between 0 (first point of polyline) and Nv-1 (last
   point of the polyline).
   

   Example
   poly = [10 10;20 10;20 20;30 30];
   polylinePoint(poly, 0)
       [10 10]
   polylinePoint(poly, 3)
       [30 30]
   polylinePoint(poly, 1.4)
       [20 14]


   See also
   polygons2d

 ------
 Author: David Legland
 e-mail: david.legland@grignon.inra.fr
 Created: 2009-04-30,    using Matlab 7.7.0.471 (R2008b)
 Copyright 2009 INRA - Cepia Software Platform.

0001 function point = polylinePoint(poly, pos)
0002 %POLYLINEPOINT Extract a point from a polyline.
0003 %
0004 %   POINT = polylinePoint(POLYLINE, POS)
0005 %   POLYLINE is a N*2 array containing coordinate of polyline vertices
0006 %   POS is comprised between 0 (first point of polyline) and Nv-1 (last
0007 %   point of the polyline).
0008 %
0009 %
0010 %   Example
0011 %   poly = [10 10;20 10;20 20;30 30];
0012 %   polylinePoint(poly, 0)
0013 %       [10 10]
0014 %   polylinePoint(poly, 3)
0015 %       [30 30]
0016 %   polylinePoint(poly, 1.4)
0017 %       [20 14]
0018 %
0019 %
0020 %   See also
0021 %   polygons2d
0022 %
0023 % ------
0024 % Author: David Legland
0025 % e-mail: david.legland@grignon.inra.fr
0026 % Created: 2009-04-30,    using Matlab 7.7.0.471 (R2008b)
0027 % Copyright 2009 INRA - Cepia Software Platform.
0028 
0029 
0030 % number of points to compute
0031 Np = length(pos(:));
0032 
0033 % number of vertices in polyline
0034 Nv = size(poly, 1);
0035 
0036 % allocate memory results
0037 point = zeros(Np, 2);
0038 
0039 % iterate on points
0040 for i=1:Np
0041     % compute index of edge (between 0 and Nv)
0042     ind = floor(pos(i));
0043     
0044     % special case of last point of polyline
0045     if ind==Nv-1
0046         point(i,:) = poly(end,:);
0047         continue;
0048     end
0049     
0050     % format index to ensure being on polyline
0051     ind = min(max(ind, 0), Nv-2);
0052     
0053     % position on current edge
0054     t = min(max(pos(i)-ind, 0), 1);
0055     
0056     % parameters of current edge
0057     x0 = poly(ind+1, 1);
0058     y0 = poly(ind+1, 2);
0059     dx = poly(ind+2,1)-x0;
0060     dy = poly(ind+2,2)-y0;
0061     
0062     % compute position of current point
0063     point(i, :) = [x0+t*dx, y0+t*dy];
0064 end