Intersection angles of two lines

[alien]

Hi
I need help on how i can find the angles and point of intersection between two lines that i have drawn on the form. I am using VB.net 2005

 

[PlausiblyDamp]

Dug up a routine I originally hacked from some C++ code i found on the internet (cant remember where though)

This should find the intersection point of two lines (hasnt been tested overly though but it seemed to work at the time from what I remember)

Public Class Line

    Public Sub New(ByVal startPoint As PointF, ByVal endPoint As PointF)
        _StartPoint = startPoint
        _EndPoint = endPoint
    End Sub

    Public ReadOnly Property StartPoint() As PointF
        Get
            Return _StartPoint
        End Get
    End Property

    Public ReadOnly Property EndPoint() As PointF
        Get
            Return _EndPoint
        End Get
    End Property

    Private ReadOnly _StartPoint, _EndPoint As PointF

    Enum IntersectResult
        Parallel
        Coincident
        DoNotIntersect
        Intersect
    End Enum


    Public Shared Function Intersect(ByVal lineOne As Line, ByVal lineTwo As Line, ByRef intersection As PointF) As IntersectResult
        Dim denominator As Double = ((lineOne._EndPoint.Y - lineOne._StartPoint.Y) * (lineTwo._EndPoint.X - lineTwo._StartPoint.X)) - _
                      ((lineOne._EndPoint.X - lineOne._StartPoint.X) * (lineTwo._EndPoint.Y - lineTwo._StartPoint.Y))

        Dim numeratorOne As Double = ((lineOne._EndPoint.X - lineOne._StartPoint.X) * (lineTwo._StartPoint.Y - lineOne._StartPoint.Y)) - _
                       ((lineOne._EndPoint.Y - lineOne._StartPoint.Y) * (lineTwo._StartPoint.X - lineOne._StartPoint.X))

        Dim numeratorTwo As Double = ((lineTwo._EndPoint.X - lineTwo._StartPoint.X) * (lineTwo._StartPoint.Y - lineOne._StartPoint.Y)) - _
                       ((lineTwo._EndPoint.Y - lineTwo._StartPoint.Y) * (lineTwo._StartPoint.X - lineOne._StartPoint.X))

        If denominator = 0 Then
            If numeratorOne = 0.0 AndAlso numeratorTwo = 0.0 Then
                Return IntersectResult.Coincident
            End If

            Return IntersectResult.Parallel
        End If

        Dim a As Double = numeratorOne / denominator
        Dim b As Double = numeratorTwo / denominator

        If a >= 0.0 AndAlso a <= 1.0 AndAlso b >= 0.0 AndAlso b <= 1.0 Then

            intersection.X = lineTwo._StartPoint.X + a * (lineTwo._EndPoint.X - lineTwo._StartPoint.X)
            intersection.Y = lineTwo._StartPoint.Y + a * (lineTwo._EndPoint.Y - lineTwo._StartPoint.Y)

            Return IntersectResult.Intersect
        End If

        Return IntersectResult.DoNotIntersect

    End Function
End Class

You can call it like

Dim l1 As New Line(New Point(0, 0), New Point(100, 100))

        Dim l2 As New Line(New Point(100, 0), New Point(0, 100))

        Dim p As PointF

        Dim i As Line.IntersectResult = Line.Intersect(l1, l2, p)

Where after the function returns p will be the point of intersection - you will need to check the return value though to make sure the lines do actually intersect.

 

[alien]

Hi thanks alot for the code. although Iam still having trouble finding the angle of intersection between two lines. what Iam trying to achieve is to get the reflection of a line once it intersects with another line. Kinda like a mirror image of the line. Please help. I am using vb.net 2005

cheers

 

[snarfblam]

If you want to find the angle of the intersection then you need to study up on some trigenometry. Ill be more than glad to help if you get stuck, but you really need to find a trig website, which will explain how to get the angle of a line based on its slope. The angle of the intersection will be the difference of the angle of the lines, and the reflected angle will simply be the compliment to the angle of intersection.