This code finds where two lines intersect just by giving this function two X and Y coordinates on the two lines.
<i>If you plan to use this code in some application, take a look at how the code handles two lines that don't cross(parallel), and when you give it the same line. Also the code can't handle vertical lines.</i>



'*****
'Nick Avery
'liserdarts@yahoo.com
'Updated 14, April, 2002
'*****
Private Type TDLine
  X As Double
  Y As Double
  X2 As Double
  Y2 As Double
End Type

Private Type TDPoint
  X As Double
  Y As Double
End Type

Private Function LinesCross(Line1 As TDLine, Line2 As TDLine) As TDPoint
Dim Slope1 As Double
Dim Slope2 As Double
Dim YInt1 As Double
Dim YInt2 As Double

   'Get the slopes
       If (Line1.X2 - Line1.X) <> 0 Then
           Slope1 = (Line1.Y2 - Line1.Y) / (Line1.X2 - Line1.X)
       Else 'The slope is undefined
           If (Line2.X2 - Line2.X) = 0 Then 'Both the lines are vertical and don't cross
               LinesCross.X = 0
               LinesCross.Y = 0
           Else
               'Get Slope2 and YInt2
                   Slope2 = (Line2.Y2 - Line2.Y) / (Line2.X2 - Line2.X)
                   YInt2 = (Slope2 * Line2.X - Line2.Y) * -1
               'Claculate X and Y
                   LinesCross.X = Line1.X
                   LinesCross.Y = Slope2 * Line1.X - (Slope2 * Line2.X - Line2.Y)
           End If
           Exit Function
       End If
       If (Line2.X2 - Line2.X) <> 0 Then
           Slope2 = (Line2.Y2 - Line2.Y) / (Line2.X2 - Line2.X)
       Else 'The slope is undefined
           'Claculate X and Y
               LinesCross.X = Line2.X
               LinesCross.Y = Slope1 * Line2.X - (Slope1 * Line1.X - Line1.Y)
           Exit Function
       End If
       
   'Get the Y intercepts
       YInt1 = -(Slope1 * Line1.X - Line1.Y)
       YInt2 = -(Slope2 * Line2.X - Line2.Y)
       
       '-B = M * X -Y
               
   'Check if the lines cross
       If Slope1 = Slope2 And YInt1 <> YInt2 Then 'The lines are parallel and don't cross
           LinesCross.X = 0
           LinesCross.Y = 0
       ElseIf Round(Slope1, 2) = Round(Slope2, 2) And YInt1 = YInt2 Then 'It is the same line and they cross at every point on that line
           LinesCross.X = Line1.X
           LinesCross.Y = Line1.Y
       Else 'The lines cross some ware
           If Slope1 = 0 Then 'Line one is horizontal
               LinesCross.Y = Line1.Y
               LinesCross.X = (YInt2 - Line1.Y) / Slope2
           ElseIf Slope2 = 0 Then 'Line two is horizontal
               LinesCross.Y = Line2.Y
               LinesCross.X = (YInt2 - Line2.Y) / Slope1
           Else
               LinesCross.Y = (YInt1 * (Slope2 / Slope1) - YInt2) / ((Slope2 / Slope1) - 1)
               LinesCross.X = (LinesCross.Y - YInt1) / Slope1
           End If
       End If
End Function

Private Sub Form_Load()
Dim Line1 As TDLine
Dim Line2 As TDLine
Dim Point As TDPoint
   
   Me.AutoRedraw = True
   Me.Scale (-50, 50)-(50, -50)
   
   Line1.X = -90
   Line1.Y = 25
   Line1.X2 = 35
   Line1.Y2 = 15
   
   Line2.X = 25
   Line2.Y = 5
   Line2.X2 = -16
   Line2.Y2 = 45
   
   Point = LinesCross(Line1, Line2)
       
   
   Me.Line (Line1.X, Line1.Y)-(Line1.X2, Line1.Y2)
   Me.Line (Line2.X, Line2.Y)-(Line2.X2, Line2.Y2)

   Me.Circle (Point.X, Point.Y), 5
End Sub
/html>