CenterStageObject Class:  Tube
Subclass of:  SurfaceFormCurve
Requires reference to a Curve object

The Tube class lets you create a tube around an existing Curve object.  By 
default, the tube is circular of constant radius, but you can make it any 
shape you want, and the radius can vary along the curve.

The second parameter for the surface is used as the parameter along the 
reference curve, while the first parameter of the surface is used to 
generate the shape for the tube.  This tube shape is expressed in the 
Frenet frame of the reference curve, in in absolute coordinates.

The Tube class supports all the directives of the SurfaceFromCurve class 
(except for the Function directive), plus the following:

    Tube param func [-frame | -absolute]
    
        Here "param" specifies the parameter that forms the shape of the 
        tube, and the "func" is an expression that returns the coordinates 
        of a point on the tube, relative to a point on the reference 
        curve.  If -frame is specified (the default), then "func" should 
        produce an ordered pair that represents a position in the 
        normal-binormal plane.  If -absolute is specified, then the 
        function should produce a point in absolute coordinates (with the 
        point on the curve as the origin).
        
        For example:
        
                Tube {theta} {(2cos(theta), sin(theta))}
        
        produces an oval tube using the Frenet frame, while
        
                Tube {s} {(cos(s),sin(s),0)}
        
        produces a circular tube, but the circles all lie in the xy-plane.
        
        Note that "func" is not a TCL script, but an expression.  If you 
        need to do more complicated computations, you can call a TCL 
        procedure as follows:
        
                Tube {s} {[myTube $s]}
                
                proc myTube {s} {
                  if {$s < $pi} {let P = (2s,0,s^2)} \
                    else {let P = (s,s,2s)}
                  return $P
                } -absolute
        
        While the tube shape usually is independent of the position along 
        the curve, you may want to make your tube depend on the curve's 
        parameter as well.  In this case, you can specify two parameters, 
        and the second is the one used by the curve:
        
                Tube {s t} { (cos(t),-sin(t),
                              sin(t), cos(t)) * (sin(2s),cos(s)) }
        
        This examples forms a figure-8 tube that rotates as it moves along 
        the curve.


    Radius r
    
        This directive specifies the radius of the tube.  After the tube 
        function is computed, it is multiplied by this value in order to 
        obtain the final tube position.
        
        You can use an expression for the radius, and the expression can 
        include the parameter for the curve.  For example:
        
                Radius {sqrt(1+t^2)}
        
        where t is the parameter listed in the Function directive of the 
        reference curve.