This thread is for discussions of Intersecting Lines.

Nick.

I took your code and added the following.
1) Return boolean as to whether or not the line segments actually cross
2) If lines are parallel or coincident return false, thus x,y can be ignored
3) Otherwise x,y are the point of intersection (where they do or would intersect)

code kept simple to be less confusing, can be collapsed to be more efficient less readable.

Public Function Intersection(Line1 As LineSegment, Line2 As LineSegment) As Intersect
   'if Line1 and Line2 intersect, return Intersect as True and the X,Y point of intersection
   'otherwise, they are parallel, coincident or do not intesect, return Intersect as False
   Dim Slope1 As Double
   Dim Slope2 As Double
   Dim YInt1 As Double
   Dim YInt2 As Double
   Dim Line1Vertical As Boolean
   Dim Line2Vertical As Boolean
   Dim Line1Horizontal As Boolean
   Dim Line2Horizontal As Boolean

   'rule out simple cases
   If Line1.x1 = Line1.x2 Then Line1Vertical = True
   If Line2.x1 = Line2.x2 Then Line2Vertical = True
   If Line1.y1 = Line1.y2 Then Line1Horizontal = True
   If Line2.y1 = Line2.y2 Then Line2Horizontal = True
   If Line1Vertical And Line2Vertical Or Line1Horizontal And Line2Horizontal Then Exit Function 'false
       
   'Get the slopes, handle simple case of eiher line vertical - both cannot due to above cases
       If Line1Vertical Then
           'Calculate X and Y as if they intersect
           Slope2 = (Line2.y2 - Line2.y1) / (Line2.x2 - Line2.x1)
           Intersection.x = Line1.x1
           Intersection.y = Slope2 * Line1.x1 - (Slope2 * Line2.x1 - Line2.y1)
           'does x,y lie on both lines?
           If Between(Intersection.x, Line1.x1, Line1.x2) And _
              Between(Intersection.x, Line2.x1, Line2.x2) And _
              Between(Intersection.y, Line1.y1, Line1.y2) And _
              Between(Intersection.y, Line2.y1, Line2.y2) Then
               Intersection.Intersect = True
             End If
           Exit Function
         Else
           Slope1 = (Line1.y2 - Line1.y1) / (Line1.x2 - Line1.x1)
         End If
       If Line2Vertical Then
           'Calculate X and Y as if they intersect
           Slope1 = (Line1.y2 - Line1.y1) / (Line1.x2 - Line1.x1)
           Intersection.x = Line2.x1
           Intersection.y = Slope1 * Line2.x1 - (Slope1 * Line1.x1 - Line1.y1)
           'does x,y lie on both lines?
           If Between(Intersection.x, Line1.x1, Line1.x2) And _
              Between(Intersection.x, Line2.x1, Line2.x2) And _
              Between(Intersection.y, Line1.y1, Line1.y2) And _
              Between(Intersection.y, Line2.y1, Line2.y2) Then
               Intersection.Intersect = True
             End If
           Exit Function
         Else
           Slope2 = (Line2.y2 - Line2.y1) / (Line2.x2 - Line2.x1)
         End If
       
       If Slope1 = Slope2 Then Exit Function
       
       'Get the Y intercepts
       YInt1 = -(Slope1 * Line1.x1 - Line1.y1)
       YInt2 = -(Slope2 * Line2.x1 - Line2.y1)
       
       '-B = M * X -Y
               
       'Check if the lines cross
       If Slope1 = Slope2 And YInt1 <> YInt2 Then 'The lines are parallel
           Exit Function
         ElseIf Round(Slope1, 2) = Round(Slope2, 2) And YInt1 = YInt2 Then 'The lines are coincident
           Exit Function
         Else 'The lines cross somewhere
           If Line1Horizontal Then
               Intersection.y = Line1.y1
               Intersection.x = (YInt2 - Line1.y1) / Slope2
             ElseIf Line2Horizontal Then
               Intersection.y = Line2.y1
               Intersection.x = (YInt2 - Line2.y1) / Slope1
             Else
               Intersection.y = (YInt1 * (Slope2 / Slope1) - YInt2) / ((Slope2 / Slope1) - 1)
               Intersection.x = (Intersection.y - YInt1) / Slope1
             End If
         End If
       'does x,y lie on both lines?
       If Between(Intersection.x, Line1.x1, Line1.x2) And _
          Between(Intersection.x, Line2.x1, Line2.x2) And _
          Between(Intersection.y, Line1.y1, Line1.y2) And _
          Between(Intersection.y, Line2.y1, Line2.y2) Then
           Intersection.Intersect = True
         End If
 End Function

I have transformed your code for my purposes using Borland c++ Builder 6. Thanx for your unknowing help. You saved me ALOT of trouble. bool CalcIntersect(double xc1,double yc1,double xc2,double yc2, double xc3,double yc3,double xc4,double yc4, double &icx,double &icy) { //x and y are used for line 1, u and v for line 2 //input x1,y1 input x2,y2 input u1,v1 input u2,v2 double x1,y1,x2,y2 = 0; double u1,v1,u2,v2 = 0; double b1,b2,a1,a2,xi,yi = 0; x1 = xc1; y1 = yc1; x2 = xc2; y2 = yc2; u1 = xc3; v1 = yc3; u2 = xc4; v2 = yc4; //rule out simple cases bool Line1Vertical = false; bool Line2Vertical = false; bool Line1Horizontal = false; bool Line2Horizontal = false; //If both lines are vertical or horizontal if(x1 == x2){Line1Vertical = True;} if(u1 == u2){Line2Vertical = True;} if(y1 == y2){Line1Horizontal = True;} if(v1 == v2){Line2Horizontal = True;} if(Line1Vertical == true && Line2Vertical == true || Line1Horizontal == true && Line2Horizontal == true) { return(false); } if(Line1Vertical == true) { //Calculate X and Y as if they intersect b2 = (v2-v1)/(u2-u1); icx = x1; icy = b2 * u1 - (b2 * u1 - v1); //does x,y lie on both lines? if(icx >= x1 && icx = u1 && icx = y1 && icx = v1 && icx = x1 && icx = u1 && icx = y1 && icx = v1 && icx = x1 && icx = u1 && icx = y1 && icx = v1 && icx