Find intersection point between a vertical line and another line.

Hi everyone,

I have a simple question which I cannot solve. I have to find the intersection point between 2 straight lines but one of them is vertical (slope==Inf) so my function is not working as desired. I have use the code here below and it works perfectly but not if one of the lines is vertical. Any ideas?

AlineX=[0 0];
AlineY=[0 400];

BLineX=[1180 570];
BLineY=[-289.5 250];

p1 = polyfit(AlineX,AlineY,1);
p2 = polyfit(BLineX,BLineY,1);

%Calculating intersection
X=fzero(@(x) polyval(p1-p2,x),3);
Y=polyval(p1,X);

As you can see, there is a A line whose starting coordinate is (0,0) and endind at (0,400) and a B line that starts at (1180,-289.5) and ends at (570,250).

One of the things I tried (since the vertical line is starting at (0,0), is doing 2 opposiute diagonals, like this:

AlineX=[-1180 -570];
AlineY=[-289.5 250];

BLineX=[1180 570];
BLineY=[-289.5 250];

p1 = polyfit(AlineX,AlineY,1);
p2 = polyfit(BLineX,BLineY,1);

%Calculating intersection
X=fzero(@(x) polyval(p1-p2,x),3);
Y=polyval(p1,X);

This gives a solution but the result X is not at 0 so I don't really understand why. Any ideas?

Many thanks in advance

Answer 1

[Alan Stevens]

You could insert a conditional:

AlineX=[0 0];
AlineY=[0 400];

BLineX=[1180 570];
BLineY=[-289.5 250];

p1 = polyfit(AlineX,AlineY,1);
p2 = polyfit(BLineX,BLineY,1);

%Calculating intersection
if AlineX(1) == AlineX(2)
    X = AlineX(1);
    Y = polyval(p2,X);
else
    X=fzero(@(x) polyval(p1-p2,x),3);
    Y=polyval(p1,X);
end

[David Rojas]

Wow, it worked! Although it shows a warning:

Warning: Polynomial is badly conditioned. Add points with distinct X values, reduce the degree of the polynomial, or try centering and
scaling as described in HELP POLYFIT. 
> In polyfit (line 72)
  In intersectFunction (line 37)    

Thank you Alan!!

[Alan Stevens]

This will eliminate the warning (which refers to p1 when you have a vertical line)

AlineX=[0 0];
AlineY=[0 400];

BLineX=[1180 570];
BLineY=[-289.5 250];

p2 = polyfit(BLineX,BLineY,1);

%Calculating intersection
if AlineX(1) == AlineX(2)
    X = AlineX(1);
    Y = polyval(p2,X);
else
    p1 = polyfit(AlineX,AlineY,1);
    X=fzero(@(x) polyval(p1-p2,x),3);
    Y=polyval(p1,X);
end