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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...

intersectEdges(edge1, edge2)
function point = intersectEdges(edge1, edge2)
%INTERSECTEDGES Return all intersections between two set of edges
%
%   P = intersectEdges(E1, E2);
%   returns the intersection point of lines L1 and L2. E1 and E2 are 1-by-4
%   arrays, containing parametric representation of each edge (in the form
%   [x1 y1 x2 y2], see 'createEdge' for details).
%   
%   In case of colinear edges, returns [Inf Inf].
%   In case of parallel but not colinear edges, returns [NaN NaN].
%
%   If each input is [N*4] array, the result is a [N*2] array containing
%   intersections of each couple of edges.
%   If one of the input has N rows and the other 1 row, the result is a
%   [N*2] array.
%
%   See also:
%   edges2d, intersectLines
%
%   ---------
%   author : David Legland 
%   INRA - TPV URPOI - BIA IMASTE
%   created the 31/10/2003.
%

%   HISTORY
%   19/02/2004 add support for multiple edges.
%   15/08/2005 rewrite a lot, and create unit tests
%   08/03/2007 update doc
%   21/09/2009 fix bug for edge arrays (thanks to Miquel Cubells)
%   03/08/2010 fix another bug for edge arrays (thanks to Reto Zingg)
%   20/02/2013 fix bug for management of parallel edges (thanks to Nicolaj
%   Kirchhof)

%% Initialisations

% ensure input arrays are same size
N1  = size(edge1, 1);
N2  = size(edge2, 1);

% ensure input have same size
if N1~=N2
    if N1==1
        edge1 = repmat(edge1, [N2 1]);
        N1 = N2;
    elseif N2==1
        edge2 = repmat(edge2, [N1 1]);
    end
end

% tolerance for precision
tol = 1e-14;

% initialize result array
x0  = zeros(N1, 1);
y0  = zeros(N1, 1);


%% Detect parallel and colinear cases

% indices of parallel edges
%par = abs(dx1.*dy2 - dx1.*dy2)<tol;
par = isParallel(edge1(:,3:4)-edge1(:,1:2), edge2(:,3:4)-edge2(:,1:2));

% indices of colinear edges
% equivalent to: |(x2-x1)*dy1 - (y2-y1)*dx1| < eps
col = abs(  (edge2(:,1)-edge1(:,1)) .* (edge1(:,4)-edge1(:,2)) - ...
            (edge2(:,2)-edge1(:,2)) .* (edge1(:,3)-edge1(:,1)) ) < tol & par;

% Parallel edges have no intersection -> return [NaN NaN]
x0(par & ~col) = NaN;
y0(par & ~col) = NaN;


%% Process colinear edges

% colinear edges may have 0, 1 or infinite intersection
% Discrimnation based on position of edge2 vertices on edge1
if sum(col) > 0
    % array for storing results of colinear edges
    resCol = Inf * ones(size(col));

    % compute position of edge2 vertices wrt edge1
    t1 = edgePosition(edge2(col, 1:2), edge1(col, :));
    t2 = edgePosition(edge2(col, 3:4), edge1(col, :));
    
    % control location of vertices: we want t1<t2
    if t1 > t2
        tmp = t1;
        t1  = t2;
        t2  = tmp;
    end
    
    % edge totally before first vertex or totally after last vertex
    resCol(col(t2 < -eps))  = NaN;
    resCol(col(t1 > 1+eps)) = NaN;
        
    % set up result into point coordinate
    x0(col) = resCol(col);
    y0(col) = resCol(col);
    
    % touches on first point of first edge
    touch = col(abs(t2) < tol);
    x0(touch) = edge1(touch, 1);
    y0(touch) = edge1(touch, 2);

    % touches on second point of first edge
    touch = col(abs(t1-1) < tol);
    x0(touch) = edge1(touch, 3);
    y0(touch) = edge1(touch, 4);
end


%% Process non parallel cases

% process edges whose supporting lines intersect
i = find(~par);

% use a test to avoid process empty arrays
if sum(i) > 0
    % extract base parameters of supporting lines for non-parallel edges
    x1  = edge1(i,1);
    y1  = edge1(i,2);
    dx1 = edge1(i,3) - x1;
    dy1 = edge1(i,4) - y1;
    x2  = edge2(i,1);
    y2  = edge2(i,2);
    dx2 = edge2(i,3) - x2;
    dy2 = edge2(i,4) - y2;

    % compute intersection points of supporting lines
    delta = (dx2.*dy1 - dx1.*dy2);
    x0(i) = ((y2-y1).*dx1.*dx2 + x1.*dy1.*dx2 - x2.*dy2.*dx1) ./ delta;
    y0(i) = ((x2-x1).*dy1.*dy2 + y1.*dx1.*dy2 - y2.*dx2.*dy1) ./ -delta;
        
    % compute position of intersection points on each edge
    % t1 is position on edge1, t2 is position on edge2
    t1  = ((y0(i)-y1).*dy1 + (x0(i)-x1).*dx1) ./ (dx1.*dx1+dy1.*dy1);
    t2  = ((y0(i)-y2).*dy2 + (x0(i)-x2).*dx2) ./ (dx2.*dx2+dy2.*dy2);

    % check position of points on edges.
    % it should be comprised between 0 and 1 for both t1 and t2.
    % if not, the edges do not intersect
    out = t1<-tol | t1>1+tol | t2<-tol | t2>1+tol;
    x0(i(out)) = NaN;
    y0(i(out)) = NaN;
end


%% format output arguments

point = [x0 y0];

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