Work with Spline IK constraints

Use Spline IK constraints to manipulate a chain of skeleton bones, such as an animal tail or a rope. You can use the constraint's default curve or use a custom spline curve such the 3D Curve from the Elements folder of the Asset Browser. Your skeleton joint chain must be exactly perpendicular to the Y-axis, otherwise, the Spline IK constraint cannot be created.

Constrain bones using a Spline IK constraint

1. In the scene, select the bones you want to constrain.
2. In the Constraints folder of the Asset Browser, select the Spline IK icon and drag it into the scene.

   The Constraints Settings tab now displays in the Navigator.

3. Drag the first bone of your skeleton to the First Joint field of the Constraint Settings.
4. Drag the last bone to the End Joint field.
5. Click the Setup tab and insert a value for Number of control nodes.

   MotionBuilder creates evenly spaced control nodes along the constrained skeleton. You use the control nodes to manipulate the curve and constrained skeleton bones.

6. Click the Constraint Settings and click Snap to constrain the bones to the curve and activate the constraint.

   A Skeleton node_3DCurve automatically populates the Spline field in the Constraint Settings.

   You can now manipulate the control nodes to translate the constraint curves and the constrained skeleton bones.

See Set up the control nodes and twist options below.

Set up the Spline IK constraint with a custom 3D curve

1. Either import a custom 3D spline curve into your scene or create one by using the 3D Curve in the Elements from the Asset Browser.

   See Create a 3D curve.

2. In the Constraints folder of the Asset Browser, select the Spline IK icon and drag it into the scene.

   The Constraints Settings tab now displays in the Navigator.

3. Complete steps 3 and 4 from the Constrain bones using a Spline IK constraint procedure above.
4. In the scene, select and drag the 3D curve to the Spline field of the Constraint Settings.
5. Click Snap to constrain the skeleton to the 3D curve.
6. In the Scene browser, select the 3D curve you are using for the constraint.
7. To setup control nodes on the curve, do the following:
   • In the 3D curve's Properties pane, locate and expand the Viewer Options.
   • Turn on AutoControlNode.

     The curve is populated with control nodes.

   • In the Scene browser, under 3D Curve, Crtl-select the 3D Curve:Points.
   • In the Properties pane, locate and expand the Marker Settings.
   • From the Look list, select a marker shape such as Hard Cross.
   • Set Size to a value between 1000 and 500.

   You can now manipulate the control nodes to translate the constraint curves and the constrained skeleton bones.

Set up the control nodes and twist options

You can add multiple control nodes for manipulating the constraint.

To add control nodes

1. In the Constraint Settings, turn off Activate.
2. Inset a value for Number of control nodes.
3. Turn on Lock control nodes.
4. Select the desired constraint Twist settings, and then click the Constraint Settings tab.

   See Spline IK constraint settings.

5. Turn on Activate.
6. Select the desired settings for Control node twist axis, Reverse twist, and Twist interpolation.