Option Explicit

'------------------------------------------------------------------------------
' Subroutine: XParallelPlane
' Purpose:    Rotate a plane about it's z-axis so that it's x-axis
              is parallel with the world xy-plane.
' Parameters:
'             plane - A valid plane to rotate
'             dir   - Direction (True = positive, False = negative)
' Returns:
'             A valid plane if successful, Null otherwise.
'------------------------------------------------------------------------------
Function XParallelPlane(plane, dir)

  Dim xaxis, yaxis, zaxis
  XParallelPlane = Null 'default return value

  zaxis = Rhino.VectorUnitize(plane(3))
  If (dir = True) Then
    xaxis = Rhino.VectorUnitize(Array(zaxis(1), -zaxis(0), 0.0))
  Else
    xaxis = Rhino.VectorUnitize(Array(-zaxis(0), zaxis(1), 0.0))
  End If

  yaxis = Rhino.VectorCrossProduct(zaxis, xaxis)
  If IsArray(yaxis) Then
    yaxis = Rhino.VectorUnitize(yaxis)
    XParallelPlane = Rhino.PlaneFromFrame(plane(0), xaxis, yaxis)
  End If

End Function