Random line segments confined in a box

Hello everyone!

Does anyone know :

How can I create random line segments confined in a box ? Both of the edges of the line segments should touch the sides of the box.

[Jan]

2D or 3D?

Answer 1

[Jan]

figure;
axes('XLim', [-0.1, 1.1], 'YLim', [-0.1, 1.1]);
nLine = 100;
Coor = [0, 1, 1, 0, 0; ...
        0, 0, 1, 1, 0];
for k = 1:nLine
    edge1 = randi(4);
    edge2 = randi(4);
    P1    = Coor(:, edge1) + rand * (Coor(:, edge1 + 1) - Coor(:, edge1));
    P2    = Coor(:, edge2) + rand * (Coor(:, edge2 + 1) - Coor(:, edge2));
    line([P1(1), P2(1)], [P1(2), P2(2)]);
end

With this code the start and endpoint can be of the same edge. If you want different edges:

    edge = randperm(4, 2);
    edge1 = edge(1)
    edge2 = edge(2);

[Adam Danz]

Nice one!

By collecting the slopes within the loop using m(k)=diff(P2)/diff(P1) there's a nice gaussian distribution of slopes, histogram(m(abs(m)<10)) (10000 iterations).

[lena kappa]

thank you guyss!!!

[lena kappa]

Hi again guys!!!

Do you know how can i find all the points where each line intersects with any other line?

[Adam Danz]

1. Look up the math for finding the intersection of two lines - it's not very difficult and there are lots of webpages that walk you through those steps.

2. Then alter Jan's solution by saving the P1 and P2 values,

P1 = nan(2, nLine);
P2 = P1; 
for k = 1:nLine
    . . .
    P1(:,k) = . . .
    P2(:,k) = . . .
    line([P1(1,k), P2(1,k)], [P1(2,k), P2(2,k)]);
end

3. Now loop through each set of coordinates to determine the intersection of line n with line j.

[lena kappa]

I tried your code but i get an error

figure;
axes('XLim', [-0.1, 1.1], 'YLim', [-0.1, 1.1]);
nLine = 252;
Coor = [0, 1, 1, 0, 0; ...
        0, 0, 1, 1, 0];
    
P1 = nan(2, nLine);
P2 = P1; 
for k = 1:nLine
    edge = randperm(4, 2);
    edge1 = edge(1);
    edge2 = edge(2);
    P1(:,k)  = Coor(:, edge1) + rand * (Coor(:, edge1 + 1) - Coor(:, edge1));
    P2(:,k) = Coor(:, edge2) + rand * (Coor(:, edge2 + 1) - Coor(:, edge2));
    fig= line([P1(k,1), P2(k,1)], [P1(k,2), P2(k,2)])  
   %[xi,yi] = polyxpoly(P1(k,1),P2(k,1),P1(k,2),P2(k,2));
end

and i get the next error:

Index in position 1 exceeds array bounds (must not exceed 2).

Error in code (line 19)

fig= line([P1(k,1), P2(k,1)], [P1(k,2), P2(k,2)])

[Adam Danz]

Oops, I corrected that error in my previous comment.

use this line instead,

line([P1(1,k), P2(1,k)], [P1(2,k), P2(2,k)])

By the way, that was a fairly simple error and the error message hints at the problem. If you try to understand the error and investigate it, you'll learn a lot very quickly.

[lena kappa]

Thanks for the code and the tip.

[lena kappa]

hey adam sorry to bother you again but i wrote this line to get the intersections and i get empty x and y

do you know why??

[xi,yi] = polyxpoly(P1(1,k),P2(1,k),P1(2,k),P2(2,k));

[george korris]

@Jan Sorry to bother you but how would you do this in 3d?

[Jan]

See here: https://www.mathworks.com/matlabcentral/answers/1818095-random-lines-confined-in-a-3d-box