Biped's inverse kinematics
solution has three parameters set at each key of the arm and leg
As the limb moves through
- IK Blend sets
the motion interpolation to be a blend of forward and inverse kinematics.
This allows you to blend swinging motions with hand-or-foot directed
motions. Rotating an arm to move a hand is an example of forward
kinematics. Using the position of the hand to move the arm is an
example of inverse kinematics. The default blend is 0.0, or full
forward kinematics. An IK Blend of 1.0 is full inverse kinematics.
- Body or Object determines the reference
coordinate space of the IK path. This allows you to move the IK
path with your character's body or temporarily attach the hands
or feet to follow other objects. The default is Body.
- Join To Prev IK Key determines
if the key should be part of the previous key (and have the same
reference position as the previous key).
Biped has three automatic ways to set these
parameters. Setting planted, sliding, or free keys creates keys
with different combinations of these three parameters.
- Setting a planted key creates a key with
IK Blend set to 1.0, Object and Join To Prev IK Key turned on.
- Setting a sliding key creates a key with
IK Blend set to 1.0, Object turned on, and Join To Prev IK Key turned
- Setting a free key creates a key with
IK Blend set to 0, and Body turned on. Join To Prev IK Key is turned
The IK Blend control is in the
Info rollout; visible when you expand
the IK divider bar. You set IK Blend while in Keyframe mode .
IK group parameters
IK Blend can be set per key for any arm or leg
track. The IK Blend setting determines whether, at a particular
key, an arm or leg is moving through it using inverse kinematics,
forward kinematics, or a blending of the two kinematic solutions.
An IK Blend value of 0.0 means full forward
kinematics. The arm (or leg) is moved by interpolating the rotations
of the joints at the keys. The hand (or foot) tends to move along
sweeping circular arcs in this case, and the motion appears to be
motivated by the apparent swinging at the joints.
An IK Blend value of 1.0 means full inverse
kinematics with the hand (or foot) being used as an end-effector.
A spline path is computed through the keys of the hand, and the
hand moves along that spline. Joint angles for the rest of the arm
are computed to allow the hand to follow the spline. The motion,
in this case, appears to be directed by the hand (or foot).
An IK Blend value between 0.0 and 1.0 means
a combination of inverse and forward kinematics; when IK Blend is
closer to 0.0, forward kinematics are more heavily weighted in the
solution, and when IK Blend is closer to 1.0, inverse kinematics
are more heavily weighted.
It is best to use forward kinematics when you
want the arms to swing, such as when a biped is walking. In the
case of a boxer, however, since the hand should follow a directed
path when punching, inverse kinematics should be used.
To set the IK Blend value of a key:
- Select a single arm or leg
track by selecting one or more parts of a biped's arm or leg.
The IK Blend spinner and the other controls
in the IK area are enabled only when a single arm or leg track is
- Set a key if one doesn't
- Set the desired value of IK Blend.
Body and Object Options
By default, Biped calculates the kinematics
solution using the coordinate system of the biped figure's center-of-mass,
or the Body coordinate system. This means that the IK path of the
hand (or foot) translates and rotates with your character as it
moves. For example, the boxer's hand trajectory always moves relative
to the weaving, bobbing, and turning of the boxer's body.
The Object option is used for animating dynamic
links between the limbs and other objects in the scene.
The IK Blend control activates when a biped
arm or leg (hand and foot) key is current.
- 0 with Body turned on is
kinematics, or normal biped space.
- 1 with Body turned on is
kinematics, creates more straight-line
motion between biped keys.
- 1 with Object turned on, but no IK object
specified, puts the limb fully into world space. Use this to control
foot sliding in a freeform animation.
- 1 with Object turned on and an IK object
specified puts the biped limb into the coordinate space of the selected
object; the biped limb follows the specified object.
Join To Prev IK Key
This IK constraint is used to specify if a footstep
is sliding or planted. If Join To Prev IK Key is on, then the biped
foot maintains a reference position to the previous key, keeping
the foot planted. If Join To Prev IK Key is off during a footstep,
then the foot can be moved to a new position creating a sliding
Join To Prev IK Key also functions to lock the
biped hands in space. Use Set Planted Key on the biped hands to
lock their position in space.
TipIf you are having
trouble with a foot or hand popping back to a previous ‘keyed’ position,
check to make sure that the Join To Prev IK Key is off, especially
when you're using pivot points.