MHX FK-IK Switch


  • FK/IK switch: These buttons show the current FK/IK status of the four limbs. Pressing the button changes from FK to IK (0% or 100% IK influence) or vice versa, and also change which bone layers are visible. 
  • IK Influence: Smoothly interpolates between full FK and full IK. Changing these sliders does not change bone layer visibility.
  • Snap Arm Bones: Snap the FK arm to the IK arm pose, or the IK arm to the FK arm pose.
  • Snap Leg Bones: Snap the FK leg to the IK leg pose, or the IK leg to the FK leg pose.
  • Switch Mode And Layers: Toggle the FK/IK mode and change bone layers after snapping.
  • Rotate IK Foot: The reverse IK foot has more degrees of freedom than the FK foot. This option determines the rotation of the IK foot after snapping.
  • Stretchiness: Add stretchiness to the arms and legs. Chaning stretchiness does not work with file linking.
  • Toes Tarsal Parents: Forgot what this is. This does not work with file linking.
The FK/IK switch buttons toggle between full FK (IK Influence = 0.0) and full IK (IK Influence = 1.0). They also toggle the visible bone layers.

With the IK Influence sliders we can interpolate between full FK and full IK. When the IK influence passes 0.5 the label of the FK/IK switch buttons change.
Eight buttons snap the FK arms and legs to their IK counterparts, or vice versa. 
The reverse foot setup of the MHX rig has some redundancy. The pose of the FK foot is determined by one location (FK foot) and two rotations (FK foot and FK toe). In contrast, the pose of the IK foot is determined by one location (IK foot) and three rotations (IK foot, reverse toe, reverse foot). Hence the rotatation of the IK foot is arbitrary. If the Rotate IK Foot option is enabled, the IK foot is rotated (left foot above), but if it is disabled the IK foot is always parallel to the ground (right foot above). The latter behaviour can be useful if we want to pin the foot to the ground in an animation.
The four stretchiness buttons determine whether the limbs can stretch or not. In the picture above the left side is stretchy, but the right side is not. There is a difference between FK and IK here. An FK limb can stretch even if the elbow or knee is bent, but an IK limb that stretches is always straightened out first.
The buttons that change stretchiness go into edit mode to change connectivity. They can hence not be invoked with file linking. To toggle stretchiness on or off can only be done in the master file.