Understanding Motion Builder’s IK/FK Control Rig Tutorial

By Ricardo Tobon

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When keying on Full Body mode and Body Part mode, Motion Builder keep the Control Rig synchronized for you. This means that FK and IK controllers move together so you can blend seamlessly between them.
However when you key on Selection mode the Control Rig can get unsynchronized quickly.
To illustrate this point make sure that Body Part is selected in the Character Controls window. Select the Left Arm FK controller and key it at frame 0 (fig 8).

Fig 8.

Go to frame 24 rotate the controller 70 degrees on the local Y axis and key frame.
Switch the Keying Mode to Selection. Go to frame 0, select the Right Arm FK controller and key it.

Go to frame 24 rotate the controller 70 degrees on the local Y axis and key frame. When you play the animation you will see both arms go down at the same time (mov 1).



Mov 1.



Go to frame 24, select the Left Wrist Effector and the Right Wrist Effector; activate their Reach T and Reach R by setting them at a value of 100. You will see the Right Arm go back to the original T position while the Left Arm stays in place. When you play the animation the left arm still moves down but the right stays up (mov 2).



Mov 2.



By activating Reach T and the Reach R you are activating the IK controller. Since you keyframed the left arm in Body Part mode keys where placed in the FK components of the arm as well as the IK components keeping both controllers synchronized. In contrast, when you keyed the right arm in Selection mode you only keyed the selected bone so the IK components were never selected or moved causing the FK controllers and the IK controllers to become unsynchronized.

Linear and Arched Trajectories

Depending on which type of controllers are driving the rig (FK or IK) your character might move different between poses.
To illustrate this concept re-open the MocapGuy.fbx file to make sure that there is no previous animation on the rig and that we have the original settings on the controllers.
Make sure that you are on Body Part Keying Mode on the Character Controls Window. Go to frame 0 and key the LeftWristEffector and the RightWristEffector, go to frame 24 and pose the left and right arms so they look like figure 9, key-frame them (fig 9). You need to adjust the position of the shoulders in order to be able to achieve the desired pose.

Fig 9.

Since we have been keying on Body Part mode, every element of both arms has been key framed and the FK controllers as well as the IK controllers are synchronized.
Even if the key-framed poses are similar the movement of a limb is very different when it is being control by FK or by IK.
Select the RightWristEffector and turn on the Reach T and the Reach R values by setting them to a 100. Ctrl select the LeftWristEffector and turn on trajectories to see the difference in movement between the arms (Mov 3).


Mov 3.



Of course the left arm which is driven by FK controllers moves more natural because it moves in an arc which is one of the basic principles of animation.

Although when you are selecting elements in the Character Controls window you are basically posing your character through the use of IK controllers. But until you activate the Reach T and Reach R of a section, you will be driving its elements with the FK controllers.


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