The parent object of the biped is its center of mass, which appears as a blue octahedron near the center of the biped’s pelvis. The name of this object is the same as the root name of the biped (Bip001 by default).
The Center of Mass (COM) object is the root parent.
The center of mass is sometimes referred to as the COM.
Center of Mass Animation Tracks
Unlike other objects and biped body parts, the center of mass has three separate animation tracks, available as buttons on the Track Selection rollout:
-
Body Horizontal -
Body Vertical -
Body Turning (selected with the Body Rotation button)
You can use these buttons to select each track as well as the COM itself.
Rotating About a Different Pivot Point
You can “decouple” rotation from the COM, and rotate the biped about a pivot point you choose. For instructions on how to do so, see To rotate the biped about a pivot that is not the COM.
Left: Pivot point moved to be near the biped’s feet.
.
Right: Rotating the biped after moving the pivot.
Linking the Center of Mass Object
Using the Select And Link tool you can link the center of mass object to another object if you need to reposition an animation sequence. An example of this might be a surfer. You could create an animation of a biped running up and down the surfboard, hanging its toes off the end of the surfboard, and so on, and then link the center of mass object to the board. When you animate the surfboard, the biped animation would travel with it.
The biped moves with the surfboard because the center of mass is linked to the surfboard.
Because the COM is the root object in the biped hierarchy, it is the only object in the biped that you can link directly to other objects in the scene by using Select And Link. To make other body parts, such as feet and hands, follow other objects in the scene, use the technique of IK attachment: see Animating IK Attachments.
Center of Mass Shadow
The center of mass shadow object, the circle between the biped’s feet on the world plane, provides a sense of the position of the character’s center of mass relative to the feet.
The center of mass shadow between the biped's feet.