How can I sort intersections points given from polyxpoly according to order of appearence?

Hi,

I'd like to find the points of intersections between two polyline using polyxpoly. The problem is that I find points in the wrong order instead of order of appearence.

This is the code I used.

[xi,yi,ii] = polyxpoly(intersections(p,1),intersections(p,2),x_pattern,y_pattern);

For a better explanation I post a picture of the problem, where I wrote the desiderd order.

Answer 1

[Bruno Luong]

Some modifications based on this FEX

x = cumsum(rand(1,10));
% Generate random P1 and P2
x = x/max(x);
y = rand(size(x));
P1 = [x; y];
y = cumsum(rand(1,10));
y = x/max(x);
x = rand(size(y));
P2 = [x; y];
X = poly2poly(P1, P2);

% Check with graphical output
figure
hold on
plot(P1(1,:),P1(2,:))
plot(P2(1,:),P2(2,:))
for k=1:size(X,2)
    plot(X(1,k),X(2,k),'o');
    text(X(1,k),X(2,k),num2str(k));
end

function X = poly2poly(P1, P2)
% function X = poly2poly(P1, P2)
% Intersection of two 2D polygons P1 and P2.
%
% INPUTS:
%   P1 and P2 are two-row arrays, each column is a vertice
%   They might or might not be wrapped around
% OUTPUT:
%   X is two-row array, each column is an intersecting point
%
% Author: Bruno Luong <[e-mail]>
% History:
%     Original 20-May-2010
% Wrap around: Pad the first point to the end if necessary
% if ~isequal(P1(:,1),P1(:,end))
%     P1 = [P1 P1(:,1)];
% end
% if ~isequal(P2(:,1),P2(:,end))
%     P2 = [P2 P2(:,1)];
% end
% swap P1 P2 so that we loop on a smaller one
if size(P1,2) > size(P2,2)
    [P1 P2] = deal(P2, P1);
end
% We increment the intermediate results by this amount
increment = 10;
% Empty buffer
X = zeros(2,0);
filled = 0;
sizec = 0;
% Loop over segments of P1
for n=2:size(P1,2)
    cn = seg2poly(P1(:,n-1:n), P2);
    m = size(cn,2);
    filled = filled+m;
    % Buffer too small
    if sizec < filled
        sizec = filled+increment;
        X(2,sizec) = 0;
    end
    % Store the result
    X(:,filled+(-m+1:0)) = cn;
end
% remove the tail
X(:,filled+1:end) = [];
end % poly2poly
%%
function X = seg2poly(s1, P)
% function X = seg2poly(s1, P)
% Check if a line segment s1 intersects with a polygon P.
% INPUTS:
%   s is (2 x 2) where
%     s(:,1) is the first point
%     s(:,2) is the the second point of the segment.
%   P is (2 x n) array, each column is a vertices
% OUTPUT
%   X is (2 x m) array, each column is an intersecting point
%
%   Author: Bruno Luong <[e-mail]>
%   History:
%       Original 20-May-2010
% Translate so that first point is origin
a = s1(:,1);
M = bsxfun(@minus, P, a);
b = s1(:,2)-a;
% Check if the points are on the left/right side
x = [b(2) -b(1)]*M;
sx = sign(x);
% x -coordinates has opposite signs
ind = sx(1:end-1).*sx(2:end) <= 0;
if any(ind)
    ind = find(ind);
    % cross point to the y-axis (along the segment)
    x1 = x(ind);
    x2 = x(ind+1);
    d = b.'/(b(1)^2+b(2)^2);
    y1 = d*M(:,ind);
    y2 = d*M(:,ind+1);
    dx = x2-x1;
    % We won't bother with the degenerate case of dx=0 and x1=0
    y = (y1.*x2-y2.*x1)./dx;
    % Check if the cross point is inside the segment
    ind = y>=0 & y<1;
    if any(ind)
        y = sort(y(ind));
        X = bsxfun(@plus, a, b*y);
    else
        X = zeros(2,0);
    end
else
    X = zeros(2,0);
end
end % seg2poly

[Matteo Nicoletta]

It works!

Thank you very much!

Answer 2

[KSSV]

I would suggest to use this.

https://in.mathworks.com/matlabcentral/fileexchange/22441-curve-intersections

[Matteo Nicoletta]

Thanks for the answer.

That function, as polyxpoly function, gives back this:

But what I want is this: