9

906 Chapter 14: character studio
onthe toes. Pivots are essentially extensions of the
IK chain.
By setting a planted key for the hand , the hand is
anchored in world space: you can move the biped
or the collarbone and the hand remains planted.
Pivots on the hands make it easy to animate hands
and fingers.
While pivots are used in both f reeform and
footstep animation, the pivots are only visible and
accessiblewheninFreeformmode.
Biped IK
To understand interactive IK limb manipulation
using character studio,itisusefultodistinguish
two typ es of limb joints: primary and secondary.
The primary joints are the shoulder, elbow,
hip, and knees. Animals use pr imary joints to
coordinate the positioning of hands and feet, as
these joints 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, ank les, 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
thesejointshavelittleinfluenceonendeffector
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 behav ior of
"natural" systems. The primary joints are used
for IK positioning, while the secondary joints are
independentlyandpreciselycontrolledbythe
animator. The secondary joints will not rotate
unless the a nimator explicitly selects and rotates
them. Therefore, if an IK pivot is set, and the
limb is interactively m anipulated, (say, the finger
is moved onto the nose), only the primary joints
adjust to satisfy t he IK constraint. The secondary
joints remain in place and will never "drift" in an
IKsolution;theysimplyobeythevaluessetby
theanimator. Becausethereisnodriftforthe
secondary joints, the animator do es not have to
tediously assign "tension" v alues to hold t hem in
place.
If an IK pivot point constraint has been set, some
by-products of character studio "natural IK" are
that:
• Interact ively 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 t he k nee about the axis from the hip
totheankle.Theserotationsgivetheanimator
a direct way to interactively rotate the primary
joints while holding the IK pivot in place.
• Interact ively translating a limb p art to move the
IK pivot will alter only the primary joints.
• Interactively rotating a primar y joint w ill move
the IK pivot with it.
After an IK pivot has been placed, apply ing 1) and
2) gives the animator exact control over all possible
IK s olutions. The IK pivots can be easily moved by
applying 3) and 4). In general, you don’t need to
remember these "r u les," because the system works
in an intuitive wa y. You merely need to position
the pivots as desired, set the IK constraint, and
thenfine-tunethepostureofthelimb,ifdesired,
by rotating the secondary joints and using t he
special rotation to adjust the primary joints.
Note: During animation play b ack, the Ankle
Tension parameter is used to set the relative