// get the transform of the control object 
var inXf = Inlocal1.value.transform;  
 
// get the control object's rotation 
var inRot = XSIMath.CreateRotation(); 
inXf.GetRotation(inRot);   

// get the value of the operator's roll parameter 
var roll = In_UpdateContext.parameters("roll").value;   

// get the value of the operator's distance parameter 
var distance = In_UpdateContext.parameters("distance").value;   

// set a vector which is going to start out as the y axis 
var yAxis = XSIMath.CreateVector3(); 
yAxis.Set(0,1,0);   

// multiply y by the rotation, so it points along the 
// control object's y axis 
yAxis.MulByRotationInPlace(inRot);   

// create the output transform of the curve 
var xf = XSIMath.CreateTransform();   

// if there is any rotation (if y doesn't point straight up) 
if(yAxis.x!=0||yAxis.z!=0) 
{ 
	// store the height of y temporarily 
	var yAxisHeight = yAxis.y;   

	// remove the y part of yAxis 
	yAxis.y = 0;   

	// normalize it (puts its length to 1) 
	yAxis.NormalizeInPlace();   

	// multiply the length of y by the distance param 
	yAxis.ScaleInPlace(distance );   

	// get the curve's geometry 
	var geo = Incrvlist.value.Geometry;   

	// get the closest curve position to the resulting 
	// position of the yAxis (on the ground plane) 
	var vbArgs = new VBArray(geo.GetClosestCurvePosition2(yAxis)); 
	var args = vbArgs.toArray();   

	// change the length of y relatively to the distance 
	// of the closest point on the curve 
	// this removes the unwanted jittery behaviour 
	yAxis.NormalizeInPlace(); yAxis.ScaleInPlace(args[3].length());   

	// now get the closest position again 
	var vbArgs = new VBArray(geo.GetClosestCurvePosition2(yAxis)); 
	var args = vbArgs.toArray();   

	// get the U value of the curve for the closest point 
	var curveU = args[2];   

	// now evaluate the curve for the given U 
	// note: this is using the hard coded subcurve index 
	// 0, so it never works for multi-curve-curvelists 
	var vbArgs = new VBArray(geo.curves(0).EvaluatePosition(curveU)); 
	var args = vbArgs.toArray();   

	// get the position and tangent 
	var curvePos = args[0]; 
	var curveTan = args[1];   

	// calculate the angle between the former Y axis and 
	// the ground plane 
	var angle = Math.acos(yAxisHeight)/2;   

	// negate (turn around) the yAxis 
	// as we want to move the object position 
	// relatively to the position on the curve 
	curvePos.NegateInPlace();   

	// define a quaternion for the rotation 
	var quat = XSIMath.CreateQuaternion();   

	// the rotation for the tilt is a rotation around 
	// the tangent of the curve on the given position 
	// with an angle defined by the overall rotation 
	// of the control object 
	quat.Set( 
		Math.cos(angle), 
		Math.sin(angle) * curveTan.x, 
		Math.sin(angle) * curveTan.y, 
		Math.sin(angle) * curveTan.z 
	);   

	// create a rotation for the tilt 
	var rotTilt = XSIMath.CreateRotation(); 
	rotTilt.SetFromQuaternion(quat);   

	// now we rotation the position of the curveobject 
	// around the tangent of the curve, by using 
	// our rotTilt rotation 
	var centerPos = XSIMath.CreateVector3(); 
	centerPos.MulByRotation(curvePos,rotTilt);   

	// define an additional rotation for the roll 
	var rotRoll = XSIMath.CreateRotation(); 
	// define an eulerangles vector for the rotation 
	var rotAngles = XSIMath.CreateVector3(); 
	// set the y rotation by the given roll param 
	rotAngles.y = XSIMath.DegreesToRadians(roll); 
	// set the rotations euler angles by the vector 
	rotRoll .SetFromXYZAngles(rotAngles); 
	// rotate the position of the curveobject again 
	// by the rotRoll rotation 
	centerPos.MulByRotationInPlace(rotRoll);   

	// for the output transform, create a new rotation 
	// and multiply both rotations together 
	var rot = XSIMath.CreateRotation(); 
	rot.Mul(rotTilt,rotRoll);   

	// now subtract the curvePosition off the center 
	// position, to put it back in its own space 
	centerPos.SubInPlace(curvePos);   
	
	// for the output transform, set the 
	// translation and rotation 
	xf.SetTranslation(centerPos); 
	xf.SetRotation(rot); 
}   

// output the transform 
Out.value.transform = xf; - See more at: http://www.softimageblog.com/archives/94#sthash.DMqooeqi.dpuf