8

Inverse Kinematics (IK) 417
Animating with Inverse
Kinematics (IK)
Using IK to animate a leg
Inverse kinematics ( IK (page 3–1052) )isamethod
of animating that reverses the direction of the
chain manipulation. Rather than work from the
root of the tree, it works from the leaves.
Let’staketheexampleofanarm. Toanimate
an arm using forward kinematics, you rotate the
upper arm away from the shoulder , then rotate
the forearm, the hand from the wrist and so, on
adding rotation keys for each child object.
To animate the arm using inverse kinematics, you
move a goal that positions the wrist. The upper
and lower ar ms are rotated by the IK solution
which moves the pivot point of t he wrist, called an
end effector (page 3–1027) ,towardthegoal.
Inthecaseofaleg,forexample,thefootis
constrained to the floor by the goal. If you move
the pelvis, the foot stays put s ince the goal has not
moved,andthiscausesthekneestobend. The
entire animation is contained in keyfr ames for the
goal and the root, without keys being applied to
the individual chain objects.
With inverse kinematics you c an quickly set
up and animate complex motions. The basic
procedure involves t hese tasks:
• Building a model. It could be a jointed st ructure
or many pieces or a single continuous surface.
• Linking the jointed model together and defini ng
pivot points, as described in
Hierarchies (page
2–398)
.
For a continuous-surface model, create a
Bone
(page 1–381)
structuretoanimatetheskinof
the char acter.
•Applying
IK solvers (page 2–421) to either the
jointed hierarchy or to the bones. You will
probably create several IK chains throughout
the hierarchy, r ather than just one. You might
also create several independent hierarchies,
rather than linking everything together in one
large hierarchy.
•Defining
joint behav ior (page 2–465) at the
pivot points, setting limits or preferred ang les,
depending on the type of IK solvers you are
using. Here you can set up sliding joints or
rotating joints. You mig ht also need to move
the root of the hierarchy, as well. You might also
want to add control objects such as dummies
at this p oint.
• Animating the go al (in the case of an
HI Solver
(page 2–427)
or IK Limb solver (page 2–454) )
or the end effector (in the case of the
HD
Solver (page 2–442)
). This animates all the
components of the IK chain.
Yo u c a n a p p l y
constraints (page 2–375) to the
goals or control objects or to the root of a chain.
Control Objects to Assist IK
Yo u c a n l i n k a g o a l o r a n e n d e f f e c t o r t o p o i n t s ,
splines, or dummy objects that serve as quick
controls to translate or rotate the end of the chain.