When this option is on, the start joint of the IK spline handle is constrained the curve. You can drag an offset manipulator to slide the start joint (and its children) along the curve.
When this option is off, you can move the start joint away from the curve. The start joint is no longer constrained to the curve. Maya ignores the Offset attribute, and no offset manipulator exists at the start joint.
This option is also available from the IK Handle Attributes.
When this option is on, a parent transform node above the start joint in the scene hierarchy is created with the spline IK handle. You can avoid unexpected start joint flipping by moving and rotating this transform node rather than the start joint. See Prevent flipping in the spline IK chain for details.You can turn on this option only when Root on Curve is off.
Auto Create Root Axis is exclusive to the IK Spline Handle Tool settings.
If the start joint of the spline IK handle has a parent, this setting makes the spline IK NURBS curve a child of that parent. Therefore, the curve and joint chain moves with the transformations of the parent.
If you create a handle that starts at a joint in the chain lower than the root joint of your skeleton, turn this option on to ensure that the joint chain moves with the transformations of its parent joint.
Auto Parent Curve is exclusive to the IK Spline Handle Tool settings.
When this option is on, the start of the curve snaps to the position of the start joint. The joints in the chain rotate to adapt to the shape of the curve.
Snap Curve to Root affects the spline IK handle only when Auto Create Curve is off in the tool settings and you already created your own curve.
Turn Snap Curve to Root off if you want to use the curve as a fixed path for your joint chain
Snap Curve to Root is exclusive to the IK Spline Handle Tool settings.
When this option is on, Maya creates a NURBS curve when you create the spline IK handle. The curve that is created follows the path of the joint chain.
If Auto Create Curve is on and Auto Simplify Curve is off, creating the spline Ik handle creates a complicated curve that passes through all the joints in the spline IK chain.
If Auto Create Curve is on and Auto Simplify Curve is on, creating the handle automatically creates a simplified curve that has a shape similar to the joint chain. The higher the Number of Spans, the closer the curve matches the joint chain. The curve has a curve degree of 3 (cubic).
If the spline IK joint chain is part of an existing skeleton, you should turn Auto Create Curve on. If you are using a curve as a path for sliding the joint chain, you should turn Auto Create Curve off.
When Auto Create Curve is off, you must create a NURBS curve for the joint chain.
Auto Create Curve is exclusive to the IK Spline Handle Tool settings.
When this option is on, Maya automatically creates a NURBS curve with the specified Number of Spans. The number of spans corresponds to the number of CVs in the curve. The curve has a curve degree of 3 (cubic).
If you create a curve with few CVs, your control of the curve’s shape and skeleton’s movement will be less precise, but you will be able to easily manipulate the curve and its joint chain. With fewer CVs, you will spend less time selecting and dragging CVs, and you are more likely to have a smooth curve.
Auto Simplify Curve is available only when Auto Create Curve is on.
Auto Simplify Curve is exclusive to the IK Spline Handle Tool settings.
Number of Spans specifies the number of CVs for the curve that Maya automatically creates when you create the spline IK handle.
The ratio between spans and CVs is as follows: number of CVs = the number of spans + 3.
Number of Spans is available only when Auto Create Curve is on.
Number of Spans is exclusive to the IK Spline Handle Tool settings.
When this option is on, manipulating the twist manipulator at the end joint will twist the start joint slightly with the other joints.
When this option is off, the start joint does not twist. If you want to turn the start joint, you have to use the roll manipulator. See Spring IK solver.
Root Twist Mode is also available in the ikHandle node’s attributes. See Root Twist Mode.
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