Maya: FK/IK Blend

In Maya motion captured animation (FK animation) and IK animation can be blended using an IK handle's attribute named ikBlend. The blending between FK and IK animations allows you to retarget a character's limbs. This operation is similar to retargeting in MotionBuilder.

Step 1. Before adding IKs to your motion capture animation, you need to check if your character has joints that you need to limit the degree of freedom of. For instance, the skeleton in Maya_joints.fbx has the following joints that need to limit the degree of freedom of: RightArmRoll, RightForeArmRoll, RightLegRoll, RightUpLegRoll, LeftArmRoll, LeftForeArmRoll, LeftLegRoll, and LeftUpLegRoll. Otherwise, the character's limbs bent at these joints with IK manipulations. To limit the degree of freedom of these joints, select a joint, open Attribute Editor (ctrl+a), open Joint tab, and:

• deselect Degree of Freedom for Y and Z. (Keep Degree of Freedom for X selected) for RightArmRoll, RightForeArmRoll, LeftArmRoll and LeftForeArmRoll.
• deselect Degree of Freedom for X and Z. (Keep Degree of Freedom for Y selected) for RightLegRoll, RIghtUpLegRoll, LeftLegRoll and LeftUpLegRoll.

RollCorrected.mb has the corrected roll joints.

Step 2. The sample skeletons' knee joints are already slightly bent, but not the elbow joints. Bent the skeleton's arms at the elbow joints slightly before adding IKs so that Maya knows in what directions the elbows should move. Elbows.mb has the corrected elbow joints.

Step 3. Add IKs. The character in IKadded.mb has IKs.

Step 4. Bind the character's skin to the skeleton.

Step 5. Export the skeleton to MotionBuilder, map and plot a motion to the skeleton, and export the skeleton. See MotionBuilder: Character.

Step 6. In Maya merge the skeleton with the Maya scene file that contains the rigged and skin-bound character. You will see a FBX import warning message about locked attributes. Select the Keep All Locked option. You may open IKadded.mb and merge walk_skeleton.fbx.

Step 7. Move in time and find the frame in which you want to start the retargeting. Select the IK handle of the limb that you want to retarget. Set 0.0 to ikBlend. Key ikBlend and the position of the IK handle.

Step 8. Move forward in time and find to the frame in which the IK should reach the target. Set 1.0 to ikBlend and move the IK to the desired position. Key ikBlend and the position of the IK.

Step 9. Move forward in time and find the frame in which you want to end the retargeting. Set 0.0 to ikBlend. Select Skeleton -> Enable IK Handle Snap a couple of times so that the IK handle snaps to the end joint's position. Key ikBlend and the position of the IK.

Step 10. Edit the ikBlend channel in Graph Editor if needed.

Notes:

• Selecting Skeleton -> Disable Selected IK Handles sets 0 to ikBlend and keys it. If ikBlend is already 0 when Skeleton -> Disable Selected IK Handles is selected, the position of the IK handle that is snapped to the end joint's position is keyed.
• Selecting Skeleton -> Enable Selected IK Handles sets 1 to ikBlend and keys it.