Spline IK Constraint¶

The Spline IK constraint aligns a chain of bones along a curve. By leveraging the ease and flexibility of achieving aesthetically pleasing shapes offered by curves and the predictability and well-integrated control offered by bones, Spline IK is an invaluable tool in the riggers’ toolbox. It is particularly well suited for rigging flexible body parts such as tails, tentacles, and spines, as well as inorganic items such as ropes.

To set up Spline IK, it is necessary to have a chain of connected bones and a curve to constrain these bones to:

With the last bone in the chain selected, add a Spline IK constraint from the Bone Constraints tab in the Properties Editor.
Set the ‘Chain Length’ setting to the number of bones in the chain (starting from and including the selected bone) that should be influenced by the curve.
Finally, set Target to the curve that should control the curve.

Note

The IK constraints are special in that they modify multiple bones. For this reason, they ignore their position in the stack and always run after all other constraints on the affected bones. To apply constraints after IK, it is necessary to first copy the final transformation to a new bone chain, e.g. using Copy Transforms.

Options¶

Target: Data ID used to select the target curve.

Spline Fitting¶

Chain Length: How many bones are included in the chain.
Even Division: Ignore the relative length of the bones when fitting to the curve.
Chain Offset: Retain the offset of the root joint from the start point of the curve.

Chain Scaling¶

Use Curve Radius

Average radius of the endpoints is used to tweak the X and Z scaling of the bones, on top of the X and Z scale mode.

Y Scale Mode

Specifies how the length of a bone is scaled when it is fitted to the curve, in addition to the effects of target curve object scale and curvature.

None: The bone is reset to its rest pose length.
Fit Curve: The bones are stretched to cover the entire length of the curve.
Bone Original: The original Y axis scale of the bone is used.

XZ Scale Mode

Scaling that a bone undergoes in the other two directions.

None: Do not scale the X and Z axes.
Bone Original: Use the original scaling of the bones.
Inverse Scale: Scale of the X and Z axes is the inverse of the Y scale.
Volume Preservation: Similar to the Stretch To constraint.

Use Original Scale

Specifies that Inverse Scale or Volume Preservation should be applied on top of the original scaling of the bones, like in the Stretch To constraint.