Find intersection of two lines

This function accepts two lines and returns the point that the lines have in common.

Description

Also included is a function that allows the user to try the function in a interactive way.

The function only accepts lines with two dimensions (x,y)

• MATLAB release: MATLAB 7.7 (R2008b)

[chiranjib sur]

 

[John]

Does not give accurate intersections. Specifically, at the endpoints of lines. Perhaps due to the high precision of the points chosen since "Warning: Matrix is singular to working precision" is shown. The line intersect function by Douglas Schwartz (file ID #11837) computes the intersection accurately.

An example script showing this:

%This script tests the function "lineintersect.m" by Paulo Silva. Note
%that when line1=1 and line2=2, lineintersect.m finds the correct
%intersection BUT line1=2 ad line2=1 does not.

clear all; close all;

%4 equidistant points on perimeter of circle of radius 100. Placed at 45,
%135, 225, 315 degrees and converted to XY crds using pol2cart function. Row #1 is X-crds, Row #2 is Y-crds
ptsXY=[70.7106781186548,-70.7106781186547,-70.7106781186548;70.7106781186547,70.7106781186548,-70.7106781186547];

%plots points as red stars
hold on;
plot(ptsXY(1,:),ptsXY(2,:),'*','color','red');

%"Lines" describes how the points are connected. Here, pt 1 and pt 3 are connected. And pt 2 and pt 3 are connected.
Lines=[1,2;3,3];

%plot lines and label lines with line IDs
for ii=1:size(Lines,2)
plot(ptsXY(1,Lines(:,ii)),ptsXY(2,Lines(:,ii)));
text(linspace(ptsXY(1,Lines(1,ii)),ptsXY(1,Lines(2,ii)),10),linspace(ptsXY(2,Lines(1,ii)),ptsXY(2,Lines(2,ii)),10),int2str(ii),'Color','black','FontWeight','normal');
end

%choose two lines to test. Note that line1=1 and line2=2 finds the correct
%intersection BUT line1=2 ad line2=1 does not.
line1=2;
line2=1;

%extract XY crds
l1_XY=[ptsXY(:,Lines(1,line1)) ptsXY(:,Lines(2,line1))]; %-each line is defined by 4 coordinates: 2 X- and 2 Y-crds
l2_XY=[ptsXY(:,Lines(1,line2)) ptsXY(:,Lines(2,line2))]; %/

%look for intersection and plot intersection
[intxX,intxY]=lineintersect([l1_XY(:,1)' l1_XY(:,2)'],[l2_XY(:,1)' l2_XY(:,2)']);
plot(intxX,intxY,'ro','MarkerFaceColor','g','LineWidth',2) %this will mark the intersection point with red 'o'

[Maher HAMDI]