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Inverse Interpolation

Posted on 2006-11-01
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Last Modified: 2009-07-29
I need to find the distance down a line.  Just wondering if there is a package (or anything really) that can perform Inverse Interpolation.

Cheers.
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Question by:nebbles
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Expert Comment

by:gangwisch
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you can use the distance formula, here is a sample:
http://www.purplemath.com/modules/distform.htm
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by:nebbles
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I know 2 points of a line.  Given a distance I need to find the point along the line and not the total distance.
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xersoft earned 500 total points
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I'm not exactly sure what you are looking for but does this help?

    Private Function GetPointByLength(ByVal P1 As PointF, ByVal P2 As PointF, ByVal DistanceFromP1 As Single) As PointF
        Dim Length As New Drawing.SizeF(P1.X - P2.X, P1.Y - P2.Y)
        Dim Angle As Single = CSng(Math.Atan2(Length.Height, Length.Width))


        Dim nx As Single = CSng(Math.Cos(Angle) * DistanceFromP1)
        Dim ny As Single = CSng(Math.Sin(Angle) * DistanceFromP1)

        Dim NewPoint As New Drawing.PointF(P1.X - nx, P1.Y - ny)

        Return NewPoint
    End Function

Give it two points and the distance and it will return the point, which is the given distance from P1. You can test it by dropping a text box on a form and naming it: txtLength

Then put this code, along with the above function into the form.

    Private Sub Form_MouseMove(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles MyBase.MouseMove
        Dim gr As Drawing.Graphics = Me.CreateGraphics


        gr.Clear(Drawing.SystemColors.Control)
        Dim p1 As New Drawing.PointF(CSng(Width / 2), CSng(Height / 2))
        Dim p2 As New Drawing.PointF(e.X, e.Y)

        gr.DrawLine(Drawing.Pens.Black, p1, p2)
        DrawPoint(gr, p1, Drawing.Color.Blue)
        DrawPoint(gr, p2, Drawing.Color.Black)


        Dim NewLength As Single = CSng(Val(txtLength.Text))

        Dim aPt As PointF = GetPointByLength(p1, p2, NewLength)
        DrawPoint(gr, aPt, Drawing.Color.Red)
    End Sub
    Private Sub DrawPoint(ByVal gr As Drawing.Graphics, ByVal p As PointF, ByVal C As Drawing.Color)
        Dim s As Single = 4

        Dim b As Drawing.Brush = New Drawing.SolidBrush(C)

        gr.FillEllipse(b, New Drawing.RectangleF(CSng(p.X - s / 2), CSng(p.Y - s / 2), s, s))

        b.Dispose()
    End Sub

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Author Comment

by:nebbles
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I had to modify the code so it wouldn't draw, but otherwise everything was there.

Thanks.
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