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  1. #1
    Join Date
    Aug 2007

    vb.net point of intersection

    Hello. Im hoping someone here will be able to help me out. Iv been searching for a while now for a way to find the point of intersection between two drawn lines in vb.net and havn't found anything. The lines that are drawn will intersect at different points everytime they are generated and also they may be drawn at any angle. What im ultimately trying to achieve is a trim of the intersecting lines so they join at the point of intersection. Im quite new to vb.net so please forgive me if there is a simple answer to my question but any help is greatly appreciated.

  2. #2
    Join Date
    Nov 2003
    Portland, OR
    Try this VB translation of this algorithm (found by searching for "geometry calculate lines intersection"):
        Function LinesIntersect( _
            ByVal Line1Start As Point, _
            ByVal Line1End As Point, _
            ByVal Line2Start As Point, _
            ByVal Line2End As Point _
        ) As Point
            Dim a1, a2, b1, b2, c1, c2, denom As Integer
            Dim intersection As Point
            a1 = Line1End.Y - Line1Start.Y
            b1 = Line1Start.X - Line1End.X
            c1 = Line1End.X * Line1Start.Y - _
                Line1Start.X * Line1End.Y
            a2 = Line2End.Y - Line2Start.Y
            b2 = Line2Start.X - Line2End.X
            c2 = Line2End.X * Line2Start.Y - _
                Line2Start.X * Line2End.Y
            denom = a1 * b2 - a2 * b1
            If denom = 0 Then
                ' Lines do not intersect
                With intersection
                    .X = (b1 * c2 - b2 * c1) \ denom
                    .Y = (a2 * c1 - a1 * c2) \ denom
                End With
            End If
        End Function
    Phil Weber

    Please post questions to the forums, where others may benefit.
    I do not offer free assistance by e-mail. Thank you!

  3. #3
    Join Date
    Aug 2007
    Thanks a million .This is very helpful and very very much apreciated.I can't thank you enough.

  4. #4
    Join Date
    Aug 2007
    ok.So that worked but now iv come to intersecting a line with a circle.Iv found some examples of how to find the intersecting points but i can't follow the math.Iv also found some that are written as functions in vb.net code and return an integer where as i need it to return a point(x,y).So can anyone help me with making a function that could do this or even help me to understand the math behind finding the point.Thanks in advance for any help.

  5. #5
    Join Date
    Aug 2007
    Iv also just realised this function only works to an integer level.Even after modifying it to take singles and pointf's it still returns integers.So does anyone have a way to intersect 2 lines and return a point to the accuracy of atleast one decimal place?.Thanks in advance

  6. #6
    Join Date
    Aug 2007

    Smile to the decimal

    ok,got it sorted.I found this page http://www.developerfusion.co.uk/show/1866/

    which i modified only slightly to get this

    Function LinesIntersect( _
    ByVal Line1Start As PointF, _
    ByVal Line1End As PointF, _
    ByVal Line2Start As PointF, _
    ByVal Line2End As PointF _
    ) As PointF
    Dim Slope1 As Double
    Dim Slope2 As Double
    Dim Int1 As Double
    Dim Int2 As Double
    Dim intersection As PointF

    'Get the slopes
    If (Line1End.X - Line1Start.X) <> 0 Then
    Slope1 = (Line1End.Y - Line1Start.Y) / (Line1End.X - Line1Start.X)
    Else 'The slope is undefined
    If (Line2End.X - Line2Start.X) = 0 Then
    MsgBox("the lines do not cross")
    'Get Slope2 and YInt2
    Slope2 = (Line2End.Y - Line2Start.Y) / (Line2End.X - Line2Start.X)
    Int2 = (Slope2 * Line2Start.X - Line2Start.Y) * -1
    'Claculate X and Y
    intersection.X = Line1Start.X
    intersection.Y = Slope2 * Line1Start.X - (Slope2 * Line2Start.X - Line2Start.Y)
    End If
    Return intersection
    End If

    If (Line2End.X - Line2Start.X) <> 0 Then
    Slope2 = (Line2End.Y - Line2Start.Y) / (Line2End.X - Line2Start.X)

    Else 'The slope is undefined
    'Claculate X and Y
    intersection.X = Line2Start.X
    intersection.Y = Slope1 * Line2Start.X - (Slope1 * Line1Start.X - Line1Start.Y)
    Return intersection
    End If

    'Get the Y intercepts
    Int1 = -(Slope1 * Line1Start.X - Line1Start.Y)
    Int2 = -(Slope2 * Line2Start.X - Line2Start.Y)

    'Check if the lines cross
    If Slope1 = Slope2 And Int1 <> Int2 Then 'The lines are parallel and don't cross
    MsgBox("Error , lines do not intersect")
    Else 'The lines cross somewhere
    If Slope1 = 0 Then 'Line one is horizontal
    intersection.Y = Line1Start.Y
    intersection.X = (Int2 - Line1Start.Y) / Slope2
    ElseIf Slope2 = 0 Then 'Line two is horizontal
    intersection.Y = Line2Start.Y
    intersection.X = (Int2 - Line2Start.Y) / Slope1
    intersection.Y = (Int1 * (Slope2 / Slope1) - Int2) / ((Slope2 / Slope1) - 1)
    intersection.X = (intersection.Y - Int1) / Slope1
    End If
    End If
    Return intersection
    End Function

    this seems to work good.Hopefully it will help someone else along the way aswell.

    I take no credit at all for this code.All credit goes to Nick Avery,i just modified his work to suit my needs.Thanks Nick.

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