User Guide > Characters > Kinematics   

FK/IK Blending

When animating a bone chain there are sometimes that you want to use bone rotations to drive the motion and other times you want an Inverse Kinematics end effector to drive the motion. Normally this means deciding between using a Forward Kinematics setup [No Kinematics in Houdini] for bone rotations or a Inverse Kinematics setup for manipulation of the end effector.

To make it possible to smoothly transition between these two setup types, Houdini's IK setup has a blend attribute that can be set using an special interactive handle that is available when using the Pose tool.

Topics covered:
Setting up the FK/IK Blend
Keyframing an FK/IK Rig


Setting up the FK/IK Blend

The ability to blend between FK and IK is a default behaviour of the Inverse Kinematics setup in Houdini. You can access the blend attribute using the following method:

1. Create and Pose an IK setup

Select tab > Character Bones, place two bones into front view then RMB-click to accept. Select tab > Character > Pose, click on one of the bones then RMB-click to accept. You can now move the end effector to move the end of the chain. Press r to get the rotation handles on the bones. Because Inverse Kinematics is controlling the chain, rotations do not work.

2. Adjust the Blend Handle

RMB-click on the end effector handle and select Show Blend Handle. A new handle shows up at the bottom of the chain's axis line. Drag this handle up until it is closer to the root of the chain. The blend parameter value becomes visible as you drag and gets set to 0 at the root.

Now you can use the rotation handles on the joints to pose the bones. The blend parameter is part of the kinematics channel operation (kin_chop) that is driving the Inverse Kinematics solution. When you adjust this value using the handle, the parameter is updated on the channel operator.

3. Snapping between setups

Press t to go back to the end effector translate handle. You can see that the end effector has been rotated along with the bones. This is because of the Auto-snap IK/Rotates option on the handle. You can choose to turn this off by RMB-clicking on the handle and selecting Auto-snap IK/Rotates. Move the handle to confirm that the end effector no longer affects the chain.

When the end effector and the joint rotations have been manipulated separately, you can get them back together by RMB-clicking on the handle and selecting either Snap to IK or Snap to Rotates. Setting the Blend parameter back to 1 is another way of smoothly blend the rotated joints back to the end effector.


Keyframing an FK/IK Rig

To set keyframes on an FK/IK setup, you must key not only the translation of the end effector but also the rotation of the joints and the blend attribute on the Kin_CHOP. With the Pose tool this is easy because pressing the k key to set keyframes on the Pose tool handles will key all of the parameters listed above.

If you do not want to blend between FK and IK then you may choose to set keyframes on only the end effector's translation parameters. In this case you should use the Transform tool and press Shift-t to set keyframes on the three translation parameters.