Animation
Following
“Advanced Animation and Rendering
Techniques”
(chapter 15+16)
By
Agata Przybyszewska
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics/inverse
kinematics
Keyframing in 2D
• Skilled animator
draws frames
• Less skilled draws
inbetweens
Keyframing in 3D
• Skilled animator draws important keyframes
• Computer generates in-betweens using
interpolation
• Rigid body motion not enough…
Keyframing
• We can move points in
3D
translation, rotation
• We can set keyframes (=
parameter to interpolate):
–
–
–
–
–
–
position,
orientation,
deformation,
lights,
camera,
opacity?
Speed control
• We want a constant velocity interpolation from A
to B
• Then we can control our own velocities
• Interpolation methods
linear, cubic, polynomial
Keyframing?
• Arc length
parametrization ->
Speed curves -> control
of movement
• Problem:
uniform steps in
parameter don’t
correspond to uniform
path distances
• Solution:
parametrization by
arclength
Overview
• Keyframing
– Parametrizing by
arclength
– Rotations
– Quaternion
interpolation
• Kinematics/inverse
kinematics
Transformations
• Every 3D transformation
is a composition of
– Rotation
– Scaling
– Translation
• We can compose
transformations
• P – point of the model
P’ – point after
transformation
Homogenous coordinates
• Use homogenous
coordinates in graphics
• Use 4 by 4 matrix to
represent:
scaling, translation,
rotation
Gimbal lock
• Gimbal = device used
for holding a
gyroscope
• Illustrates problem
with interpolating
Euler angles
• Gimbal lock is a basic
problem with
representing 3D
rotations using Euler
angles
Axis Angle
• Eulers rotation theorem
Any orientation can be
represented by a 4-tuple
angle, vector(x,y,z)
where
angle – amount to rotate
vector – axis of rotation
We can interpolate angle and
axis seperately
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics/inverse
kinematics
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics
– Articulated figures
– Forward kinematics
– Inverted kinematics
Kinematics
• Study of movement
without regards to the
forces that cause it
Kinematics
• Inbetweening
– Inverse kinematics or dynamics
• Articulated figure
– Structure of rigid liks connected
at joints
• Degrees of freedom (D0F)
– number of independent
position variables
• End effector
– End of chain of links
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics
– Articulated figures
– Forward kinematics
– Inverted kinematics
Articulated figures
• Suitable for humanoid figures
• Want the structure of the body to
be maintained
• Horrible approximation
Articulated figures
• Inbetweening
– Compute joint angles between computer frames
DH notation
• Represents state of
articulated structure
• Attach coordinate
frame to each link
• Transformation matrix
between consecutive
coordinate frames
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics
– Articulated figures
– Forward kinematics
– Inverted kinematics
Forward kinematics
• Joint motion can be specified by spline curves
Forward kinematics
Example
• Walk cycle
• Hip joint orientation
Example
• Walk cycle
• Ankle joint orientation
Overview
• Keyframing
– Parametrizing by
arclength
– General kinematics
– Rotations
– Quaternion
interpolation
• Kinematics
– Articulated figures
– Forward kinematics
– Inverse kinematics
Inverse kinematics
• What if animation knows position at the end?
• Pick up object from plate, put object in place
Inverse kinematics
• End effector positions can be specified by spline
curves
Inverse kinematics
• Solution for more complex structures
– Find best solution (ie. Minimize energy in motion)
– Non linear optimization
What makes IK hard?
• Ill conditioned near
singularities
– High state velocities
for low cartesian
velocities
• Redundancies
– Add constraints to
reduce redundancies
• Find “closest”
solution:
– Minimize time
– Minimize energy
– Natural looking motion
IK and the Jacobian
•Use inverse Jacobian
•Jacobian maps velocities in state space to velocities in
cartesian space
•Iteratively step all the joint angles toward the goal
Computing IK
• Computational problems
– Singularities – change rank
– Jacobian only valid for given configuration
• Non linear optimization
– Nummerical programming:
Method for finding (local) minimum of function
Summary of kinematics
• Forward kinematics
– Specify conditions (joint angles)
– Compute positions and end-effectors
• Reverse kinematics
– “Goal directed” motion
– Specify goal positions of end effectors
– Compute conditions required to achieve
goals
• For many tasks inverse kinematics
provides easier specification, but is
more computationally difficult