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452 Chapter 12: Animation
The IK c hain is paren ted (via th e start joint) at object A.
Assume this is the pose when the IK solver is
assigned. So, this is the preferred pose. The plane
on that the joints are laid out is the horizontal
plane of the (Zero Plane Map) sphere.
• A: Parent Space is Start Joint. In this case, the
sphere is parented to A. If A is rotated about
the drawn axis, the sphere is rotated together
with it. The goal is in a separate transformation
hierarchy. It stays in place, and the end effector
sticks to it because of the IK solution. Since the
(plane) normal is fixed to the sphere, it rotates
w ith A, too. Therefore, the whole chain appears
to be rotated together with the parent object.
• B: Parent Space is IK Goal. Suppose that the
goal is parented to the world. In this case, the
sphere is parented to the world and, hence,
stays fixed. Since the normal is fixed to the
sphere, the chain will appear stationary when
Aisrotated.
Example 2
In the following example, we look at a case where
there exists a rotation in the parent space when the
IK solver is assigned.
The p are nt space of the IK chain contains a rotation when the
IK solver is assigned.
Parent A contains a rotation of 90 degrees. This is
an abst raction of t he case when a user creates four
bones without an IK solver and later assigns an IK
solver f rom Bone2 to Bone4. If we parent the chain
directly to world, it would appear as shown in the
right figure: the solver plane becomes horizontal.
• A: ParentSpaceisStartJoint. Thesphereis
mountedaftertherotationofAandtherefore
the “horizontal plane” coincides with the chain
plane as shown in the viewport. The singular
points are perpendicular to the Start Joint with
regard to the plane. When the goal/end effector
is mo ved on the plane, it will never hit the
singular points and flip.
• B: Parent Space is IK Goal. Again, we assume
that the parent space of the goal is the world.
The“horizontalplane”ofthespherebecomes
horizontal, as show n in the ri ght figure. The
singular points, the poles, are on the plane that
joints are laid out. Therefore , w hen users move
the goal/end effector left to right, or the other
around, the end effector will move across the
singular point and flip.
A problem of B is that the figure on the right is
never shown to the user . They have to envision it
in order to understand the flipping.