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geom2d

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geom2d

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13 Jun 2005 (Updated )

Geometry library for matlab. Performs geometric computations on points, lines, circles, polygons...

polygonLoops(poly)
function loops = polygonLoops(poly)
%POLYGONLOOPS Divide a possibly self-intersecting polygon into a set of simple loops
%
%   LOOPS = polygonLoops(POLYGON);
%   POLYGON is a polygone defined by a series of vertices,
%   LOOPS is a cell array of polygons, containing the same vertices of the
%   original polygon, but no loop self-intersect, and no couple of loops
%   intersect each other.
%
%   Example:
%       poly = [0 0;0 10;20 10;20 20;10 20;10 0];
%       loops = polygonLoops(poly);
%       figure(1); hold on;
%       drawPolygon(loops);
%       polygonArea(loops)
%
%   See also
%   polygons2d, polygonSelfIntersections
%

% ------
% Author: David Legland
% e-mail: david.legland@grignon.inra.fr
% Created: 2009-06-15,    using Matlab 7.7.0.471 (R2008b)
% Copyright 2009 INRA - Cepia Software Platform.


%% Initialisations

% compute intersections
[inters, pos1, pos2] = polygonSelfIntersections(poly);

% case of a polygon without self-intersection
if isempty(inters)
    loops = {poly};
    return;
end

% array for storing loops
loops = cell(0, 1);

% sort intersection points with respect to their position on the polygon
[positions, order] = sortrows([pos1 pos2;pos2 pos1]);
inters = [inters ; inters];
inters = inters(order, :);


%% First loop

% initialize the beginning of the loop
pos0 = 0;
loop = polygonSubcurve(poly, pos0, positions(1, 1));
loop(end, :) = inters(1,:);
vertex = inters(1,:);

% prepare iteration on positions
pos = positions(1, 2);
positions(1, :) = [];
inters(1,:) = [];

while true
    % index of next intersection point
    ind = find(positions(:,1) > pos, 1, 'first');
    
    % if not index is found, the current loop is complete
    if isempty(ind)
        break;
    end

    % compute the portion of curve between the two intersection points
    portion = polygonSubcurve(poly, pos, positions(ind, 1));
    
    % ensure extremities have been computed only once
    portion(1, :) = vertex;
    vertex = inters(ind, :);
    portion(end, :) = vertex;
    
    % add the current portion of curve
    loop = [loop; portion]; %#ok<AGROW>
    
    % update current position on the polygon
    pos = positions(ind, 2);
    
    % remove processed intersection
    positions(ind, :) = [];
    inters(ind,:) = [];
end

% add the last portion of curve
loop = [loop;polygonSubcurve(poly, pos, pos0)];

% remove redundant vertices
loop(sum(loop(1:end-1,:) == loop(2:end,:) ,2)==2, :) = [];
if sum(diff(loop([1 end], :))==0)==2
    loop(end, :) = [];
end

% add current loop to the list of loops
loops{1} = loop;


%% Other loops

Nl = 1;
while ~isempty(positions)

    % initialize the next loop
    loop    = [];
    pos0    = positions(1, 2);
    pos     = positions(1, 2);
    vertex  = inters(1,:);
    
    while true
        % index of next intersection point
        ind = find(positions(:,1) > pos, 1, 'first');

        % compute the portion of curve between the two intersection points
        portion = polygonSubcurve(poly, pos, positions(ind, 1));
        
        % ensure extremities have been computed only once
        portion(1, :) = vertex;
        vertex = inters(ind, :);
        portion(end, :) = vertex;
        
        % add the current portion of curve
        loop = [loop; portion]; %#ok<AGROW>

        % update current position on the polygon
        pos = positions(ind, 2);

        % remove processed intersection
        positions(ind, :) = [];
        inters(ind,:) = [];

        % if not found, current loop is processed
        if pos == pos0
            break;
        end
    end

    % remove redundant vertices
    loop(sum(loop(1:end-1,:) == loop(2:end,:) ,2)==2, :) = []; %#ok<AGROW>
    if sum(diff(loop([1 end], :))==0)==2
        loop(end, :) = []; 
    end

    % add current loop to the list of loops
    Nl = Nl + 1;
    loops{Nl} = loop;
end

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