2009

have the most influence and flexibility in positioning tasks. Even the placement
of fingers and toes over specific spots is the task of the primary joints.
The secondary joints are the wrists, ankles, toes, and fingers. These joints are
typically used for grasping and support rather than positioning, so they most
frequently exercise independent joint angle control suited to a particular task,
such as rotating the foot to raise the heels during walking, or curling the
fingers around an object. Because these joints have little influence on end
effector position, they are rarely engaged for positioning tasks. For example,
if you want to place your finger on your nose, you will most naturally rotate
only your shoulder and elbow, keeping your wrist stationary.
character studio mimics the IK behavior of "natural" systems. The primary
joints are used for IK positioning, while the secondary joints are independently
and precisely controlled by the animator. The secondary joints will not rotate
unless the animator explicitly selects and rotates them. Therefore, if an IK
pivot is set, and the limb is interactively manipulated, (say, the finger is moved
onto the nose), only the primary joints adjust to satisfy the IK constraint. The
secondary joints remain in place and will never "drift" in an IK solution; they
simply obey the values set by the animator. Because there is no drift for the
secondary joints, the animator does not have to tediously assign "tension"
values to hold them in place.
If an IK pivot point constraint has been set, some by-products of character
studio "natural IK" are that:
■ Interactively rotating a secondary joint will always adjust the primary
joints to hold the IK pivot in place.
■ character studio "special rotations," the forearm/calf X-axis rotations, rotate
the elbow about the axis from the shoulder to the wrist and rotate the
knee about the axis from the hip to the ankle. These rotations give the
animator a direct way to interactively rotate the primary joints while
holding the IK pivot in place.
■ Interactively translating a limb part to move the IK pivot will alter only
the primary joints.
■ Interactively rotating a primary joint will move the IK pivot with it.
After an IK pivot has been placed, applying 1) and 2) gives the animator exact
control over all possible IK solutions. The IK pivots can be easily moved by
applying 3) and 4). In general, you don't need to remember these "rules,"
because the system works in an intuitive way. You merely need to position
the pivots as desired, set the IK constraint, and then fine-tune the posture of
4274 | Chapter 17 character studio