Thursday, August 24, 2023

MHX and Spine IK

If we have perfected a pose for the upper body, we want it to stay that way even if we change the pose for the lower body. This has not been possible with the MHX rig before, because even if the hands are controlled by IK and stay in place, the arms and head move when we move the hip bone. In the latest version of MHX, we can add an IK controller to the spine bones, which lets us control the spine from the top instead of from the bottom. Thus the top of the spine and its children remain in place when we move the hip bone.

To add spine IK to the MHX rig, the Spine IK option must be enabled. We notice that we can also add IK controllers to other parts of the rig, namely the fingers, tongue, and the Dicktator/Futalicious shaft.

The controls are located in the MHX tab, in the panel called FK/IK Spine Fingers Tongue Shaft, If Spine IK is enabled, we pose the spine with the wiggly ik_back bone at the top of the spine. It doesn't move if we move the hip.
The Snap FK button snaps the FK back bone to match the ik_back bone, and sets Spine IK to zero. The pose remains the same for intermediate values of the spine IK influence. We can now pose the spine with the back bone instead. When the hip moves, so does the upper body.
The Snap Spine button sets the pose for the individual spine bones on the Spine layer, and resets the locations of the back and ik_back bones. Use this if you want to save a pose preset for use in DAZ Studio.
The same mechanism is used for finger IK, if we enabled that when the MHX rig was generated. We can pose the fingers with the long finger bones if the finger IK influence is zero.
The Snap IK button moves the finger IK bones to the end of finger FK bones, and sets the IK influence to one. We can now pose the fingers with the IK bones. This can be useful if we want to fingers to interact with something else.
The tongue is handled in the same way. The tongue is posed with the long tongue bone on the Head layer if Tongue IK is zero. The pose can be tweaked with the individual tongue bones on the Face layer.
The Snap IK button snaps the ik_tongue bone into position. Note how the bone shape changes when the bone is scaled. Snap Tongue snaps the individual tongue bones to their current location, and resets the FK and IK tongue.

The MHX rig can also include FK and IK bones to control the Dicktator or Futalicious shaft bones, or more generally any chain the bones starting with the name "shaft". They work in a completely analogous way.

Finally we can note that the user interface for FK/IK for arms and legs has changed. The functionality is the same as before, but the design has be changed to look the same as the other FK/IK panels.