Code covered by the BSD License  

Highlights from
Polygon_Intersection

image thumbnail

Polygon_Intersection

by

 

01 Jan 2008 (Updated )

This function computes n-times intersection region of shapes collection

Polygons_intersection.m
function [Geo] = Polygons_intersection(S,Display_result,Accuracy)
% This function computes n-times intersection region of shapes collection 
% and allows to identify every intersection region in which shapes 
% intersect.
% 
% The function takes one argument as input,  a structure S containing 
% geometrical descriptions of shapes, and delivers one output argument,
% a structure containing the different shape intersections, with the
% shape indexes involded in the intersection and the associated area.
% The second argument is optional. Display_result is a binary value which 
% indicates whether the result should be displayed (1) or not (0).
%
% Input:  S   : Structure containing geometrical description of polygons.
%         S(i) contains all information relative to the i-th shape
%         S(i).P(j) gives access to the geometrical description of the j-th
%         element of the i-th shape. 
%               XData : S(i).P(j).x    : Vector
%               YData : S(i).P(j).y    : Vector
%               Hole  : S(i).P(j).hole : Binary value (1= hole, 0= fill).
%               This binary variable indicates whether the consider polygon
%               S(i).P(j) is a hole or not. Hole are represented with
%               dotted lines on figures.
%         Display_result: Binary value used to display or not result
%         Accuracy:       Relative accuracy. The accuracy of the algorithm
%         will the product of this parameter and the area of the largest
%         polygon
%
% Output: Geo : Structure containing geometrical description of the 
%               intersection polygons, their area, and shape indexes used
%               to compute the intersection.
% 
%               Geo(i).index   contains the polygon indexes          
%               Geo(i).P       contains the geometry of the polygon
%                              P is a structure containing XData & 
%                              YData of polygons.
%                               XData : Geo(i).P(j).x    : Vector
%                               YData : Geo(i).P(j).y    : Vector
%                               Hole  : Geo(i).P(j).hole : Binary value (1
%                                                          = hole, 0= fill)
%               Geo(i).area    contains the area of the i-th shape.
%
% This function starts to evaluate the intersection between all shapes.
% Results are stored in an intersection matrix M, where M(i,j) contains the
% intersection between shape i & j.
% Then, for each shape which intersects other shapes, one lists all
% possible polygons involved in the intersection with the function
% Polygons_intersection_Create_combination. The results is a list of
% possible intersection, each element of this list is tested and each time
% there is intersection, a new shape is created with the attribute of the
% polygons involved in the intersection.
% 
% Then, new shapes that are the results of several intersection
% computations are substracted to polygons lower shape indexes.
% 
% This function uses Polygon Clipper, a function posted by Sebastian Hlz:
% 
% "The Polygon Clipper (based on the gpc-library) is used to performe
% algebraic operations on two polygons.
% Given two arbitrary polygons (which may self intersect, may contain 
% holes, may be constructed of several contours) the Polygon Clipper 
% is used to calculate the resulting polygon for the operations diff, 
% union, AND, XOR.
% 
% Credit for the gpc-library goes to Alan Murta." 
%
%
%
% Guillaume JACQUENOT
% guillaume at jacquenot at gmail dot com
% 2007_10_08
% 2009_06_16

if nargin==0
% Creates initial geometry for an example.
    S(1).P(1).x    = [1.5 1.5 0.5 1.5+cos(pi:pi/30:3*pi/2)];
    S(1).P(1).y    = [0.5 1.5 1.5 1.5+sin(pi:pi/30:3*pi/2)];
    S(1).P(1).hole = 0;
    S(1).P(2).x    = [0.7 0.9 0.9 0.7 0.7]+0.5;
    S(1).P(2).y    = [0.9 0.9 1.1 1.1 0.9]+0.27;
    S(1).P(2).hole = 1;

    S(2).P(1).x    = [0.4 0.4 1.6   1.6 1.25 0.4];
    S(2).P(1).y    = [1 0.9 0.725 0.9 1.25 1];   
    S(2).P(1).hole = 0;

    S(3).P(1).x    = [0.5 0.75 1.125 1.5 1.125 1.125 0.75 1 0.8 0.5];
    S(3).P(1).y    = [0.5 0.125 0.125 0.375 1 0.3 0.43 1.125 1.4 0.5]+0.05;
    S(3).P(1).hole = 0;
    
    Display_result = 1;
    Accuracy       = 1e-6;
    Polygons_intersection(S,Display_result,Accuracy);
    
    clear S
    S(1).P.x = [0  -10.9180   -9.8212    1.0968 0];
    S(1).P.y = [0   -1.3406  -10.2735   -8.9329 0];
    S(2).P.x = [6   -4.9180   -3.8212    7.0968 6];
    S(2).P.y = [0   -1.3406  -10.2735   -8.9329 0];
    S(3).P.x = [6   -4.9180   -3.8212    7.0968 6];
    S(3).P.y = [5    3.6594   -5.2735   -3.9329 5];
    S(4).P.x = [0  -10.9180   -9.8212    1.0968 0];
    S(4).P.y = [5    3.6594   -5.2735   -3.9329 5];    
    
    S(1).P.hole = 0;
    S(2).P.hole = 0;
    S(3).P.hole = 0;
    S(4).P.hole = 0;

    Display_result = 1;
    Accuracy       = 1e-6;
    Polygons_intersection(S,Display_result,Accuracy);
    return;
elseif nargin == 1
    Display_result = 0;
    Accuracy       = 1e-6;
elseif nargin == 2
    Accuracy       = 1e-6;
end


% Number of polygons stored in N_polygons
N_polygons = numel(S);
for i=1:N_polygons
    S(i).index = i;
end
% Compute the area of all elements
S = Polygons_intersection_Compute_area(S);

% Compute total accuracy
Accuracy = Accuracy * max([S(:).area]);

% Preallocating memory to store intersection area between polygons.
Res = zeros(N_polygons);

% Initiate index, which will used to store domains 
index = 1;

% Storing_index is used to know where the information of the intersection
% polygons i & j is stored in the output structure Geo
Storing_index = zeros(N_polygons);

% Performs a double sweep on all polygons to perform a first polygon
% intersection

% index is used to count the number of intersections
% Geo   is used to the output structure containing all intersection results
% Compute a first level intersection
for i = 1:N_polygons-1
    for j = i+1:N_polygons
        P = PolygonClip(S(i).P,S(j).P,1);
        % if the intersection is not empty
        if size(P,2)~=0
            % for all elements of P
            for p = 1:size(P,2)
              Res(i,j) =Res(i,j) + (1-2*P(p).hole)*polyarea(P(p).x,P(p).y);
            end
            Storing_index(i,j)=index;
            % Initiates Geo, structure containing all results.
            Geo(index).index  = [i,j];
            Geo(index).P      = P;
            Geo(index).area   = Res(i,j);
            index = index +1;            
        end
    end
end
index_first = index -1;

% This function creates a list of all possible overlapping elements
% computed from an intersection area matrix
Index2 = Polygons_intersection_Create_combination(Res);

V = zeros(1,numel(Index2));
for i=1:numel(Index2)
    Geo(index_first+i).index  = Index2(i).data;
    j=1;
    str2compare   = Index2(i).data(1:end-1);
    N_str2compare = numel(str2compare);
    bool=0;
    while (j<=index_first+i-1) && bool==0
        if length(str2compare)==length(Geo(j).index);
            if sum(str2compare == Geo(j).index)==N_str2compare
                bool=1;
            else
                j=j+1;
            end
        else
            j=j+1;
        end        
    end
    V(i)=j;
end

% One computes possible polygon intersections.
To_delete = [];
for i=1:numel(Index2)
    if ~isempty(Geo(V(i)).P)
        P=PolygonClip(Geo(V(i)).P,S(Index2(i).data(end)).P,1);
        if size(P,2)~=0
            area = 0;
            for p = 1:size(P,2)
                area = area + (1-2*P(p).hole)*polyarea(P(p).x,P(p).y);
                Geo(index_first+i).P    = P;
                Geo(index_first+i).area = area;
            end
        else
            To_delete = [To_delete,index_first+i];
        end
    else
        To_delete = [To_delete,index_first+i];
    end
end
Geo(To_delete)=[];

% Posttreatment
N_Geo = numel(Geo);
for i=1:N_Geo
    for j=1:numel(Geo(i).P)
        % Polygons are closed
        if ((Geo(i).P(j).x(end)~=Geo(i).P(j).x(1)) ||...
            (Geo(i).P(j).y(end)~=Geo(i).P(j).y(1)))
            Geo(i).P(j).x = [Geo(i).P(j).x;Geo(i).P(j).x(1)];
            Geo(i).P(j).y = [Geo(i).P(j).y;Geo(i).P(j).y(1)];
        end
    end
end

% N_Geo = numel(Geo);
% colour = rand(N_Geo,3);
% figure, hold on, grid on
% for i=1:N_Geo
%     for j=1:numel(Geo(i).P)
%         h = plot(Geo(i).P(j).x,Geo(i).P(j).y,'color',colour(i,:),'LineWidth',3);
%         if Geo(i).P(j).hole==1
%             set(h,'LineStyle','--');
%         end
% %         text(S(n).x(1)+0.2,S(n).y(1)+0.2,num2str(n),...
% %              'color',colour(n,:),'FontSize',14);
%     end
% end

% Now that the all intersection polygons have been computed, one calls the
% following function that will splits the original polygons into smaller
% ones
Geo = Polygons_intersection_Posttreatment(S,Geo,Display_result,Accuracy);

Contact us