CatenaryCrv.rvb

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' CatenaryCrv.rvb -- April 2005
' If this code works, it was written by Dale Fugier.
' If not, I don't know who wrote it.
' Works with Rhino 4.0.
'
' Ref: http://mathworld.wolfram.com/Catenary.html
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Option Explicit

Sub CatenaryCrv()

  Dim t0, t1, t, a
  Dim cnt, x, pt, pts()

  t1 = Rhino.GetReal("Value of X", 1.0, 0.1)
  If IsNull(t1) Then Exit Sub
    
  t0 = -t1
    
  a = Rhino.GetReal("Value of A", 1.0, 0.1)
  If IsNull(a) Then Exit Sub

  cnt = Rhino.GetInteger("Number of sampling points", 50, 3)
  If IsNull(cnt) Then Exit Sub

  ReDim pts(cnt - 1)

  pt = CalculatePoint(t0, a)
  pts(0) = pt

  For x = 1 To cnt - 2
    t = (1.0 - (x / cnt) ) * t0 + (x / cnt) * t1
    pt = CalculatePoint(t, a)
    pts(x) = pt
  Next

  pt = CalculatePoint(t1, a)
  pts(cnt - 1) = pt

  Rhino.SelectObject Rhino.AddInterpCurveEx(pts)

End Sub

Function CalculatePoint(t, a)
  Dim arr(2)
  If IsNumeric(t) Then
    arr(0) = t
    arr(1) = a * cosh(t/a)
    arr(2) = 0
    CalculatePoint = arr
  Else
    CalculatePoint = vbNull
  End If
End Function

Function Cosh(x)
  Cosh = (Exp(x) + Exp(-x)) / 2  
End Function