resamplePolylineByLength

RESAMPLEPOLYLINEBYLENGTH Resample a polyline with a fixed sampling step.

   RES = resamplePolyline(POLY, STEP)
   Resample the input polyline POLY by distributing new vertices on the
   original polyline such that the (curvilinear) distance between the new
   vertices is approximately equal to STEP. 

   Example
     poly = [0 10;0 0;10 0; 10 10; 20 10;20 0];
     figure; drawPolyline(poly, 'k');
     poly2 = resamplePolylineByLength(poly, 4);
     hold on; 
     drawPolyline(poly2, 'm');
     drawPoint(poly2, 'mo');
     axis equal; axis([-10 30 -10 20]);
     legend('Original polyline', 'Resampled polyline');

   See also
     polygons2d, drawPolyline, resamplePolygon

0001 function poly2 = resamplePolylineByLength(poly, step)
0002 %RESAMPLEPOLYLINEBYLENGTH Resample a polyline with a fixed sampling step.
0003 %
0004 %   RES = resamplePolyline(POLY, STEP)
0005 %   Resample the input polyline POLY by distributing new vertices on the
0006 %   original polyline such that the (curvilinear) distance between the new
0007 %   vertices is approximately equal to STEP.
0008 %
0009 %   Example
0010 %     poly = [0 10;0 0;10 0; 10 10; 20 10;20 0];
0011 %     figure; drawPolyline(poly, 'k');
0012 %     poly2 = resamplePolylineByLength(poly, 4);
0013 %     hold on;
0014 %     drawPolyline(poly2, 'm');
0015 %     drawPoint(poly2, 'mo');
0016 %     axis equal; axis([-10 30 -10 20]);
0017 %     legend('Original polyline', 'Resampled polyline');
0018 %
0019 %   See also
0020 %     polygons2d, drawPolyline, resamplePolygon
0021 %
0022 
0023 % ------
0024 % Author: David Legland
0025 % e-mail: david.legland@inra.fr
0026 % Created: 2011-12-09,    using Matlab 7.9.0.529 (R2009b)
0027 % Copyrightf 2011 INRA - Cepia Software Platform.
0028 
0029 % parametrisation of the curve
0030 s = parametrize(poly);
0031 
0032 % compute the number of points for sampling the polygon
0033 % (equal to the number of segments plus one)
0034 Lmax = s(end);
0035 n = round(Lmax / step) + 1;
0036 
0037 % distribute N points equally spaced
0038 pos = linspace(0, Lmax, n);
0039 
0040 poly2 = zeros(n, size(poly, 2));
0041 for i = 1:n
0042     % index of surrounding vertices before and after
0043     ind0 = find(s <= pos(i), 1, 'last');
0044     ind1 = find(s >= pos(i), 1, 'first');
0045     
0046     if ind0 == ind1
0047         % get position of a vertex in input polyline
0048         poly2(i, :) = poly(ind0, :);
0049         continue;
0050     end
0051     
0052     % position of surrounding vertices
0053     pt0 = poly(ind0, :);
0054     pt1 = poly(ind1, :);
0055     
0056     % weights associated to each neighbor
0057     l0 = pos(i) - s(ind0);
0058     l1 = s(ind1) - pos(i);
0059     
0060     % linear interpolation of neighbor positions
0061     if (l0 + l1) > Lmax * 1e-12
0062         poly2(i, :) = (pt0 * l1 + pt1 * l0) / (l0 + l1);
0063     else
0064         % if neighbors are too close, do not use interpolation
0065         poly2(i, :) = pt0;
0066     end
0067 end