SLIDE 1
Motion Capture
CS418 Interactive Computer Graphics John C. Hart
SLIDE 2 Flexible Body Animation
- Need same number and configuration of
vertices at key frames for intervening frames to make sense
- Need to have correspondences between two
collections of vertices Motion Capture
- Place fiducial markers (e.g. ping pong
balls) on a real-world object
- Capture 3-D pose of markers at key frames
- Use motion of markers to deform model
A motivating example from: Sederberg & Greenwood, A Physically- Based Approach to 2-D Shape Blending, Proc. SIGGRAPH 92
SLIDE 3
Place Fiducial Markers
SLIDE 4 Create Bone Model
Endpoint positions based
- n geometric combinations
- f fiducial marker positions
SLIDE 5
Create Bone Model
SLIDE 6
Measure Joint Angles
θ3 θ4 θ1 θ2
SLIDE 7
Fit New Pose
θ3 θ4 θ1 θ3 θ1 θ2 θ2 θ4
SLIDE 8
Joint Angles = Pose
θ3 θ4 θ1 θ3 θ1 θ2 θ2 θ4
SLIDE 9
Model Shape from Bones
θ1 θ2 θ3 θ4 θ1 θ3 θ2 θ4
SLIDE 10
Model Shape from Bones
θ1 θ2 θ3 θ4 θ1 θ2 θ3 θ4
SLIDE 11
Motion Retargeting
θ1 θ2 θ3 θ4 θ1 θ2 θ3 θ4
SLIDE 12
Simple Inverse Kinematics
Given target point (x,y) in position space, what are the parameters (θ,φ) in configuration space that place the hand on the target point? (0,0) φ a b (x,y) θ
SLIDE 13
Simple Inverse Kinematics
Use Law of Cosines to find θ d2 = a2 + b2 – 2ab cos θ cos θ = (a2 + b2 – d2)/2ab cos θ = (a2 + b2 – x2 – y2)/2ab (0,0) α b (x,y) θ a d
SLIDE 14
Simple Inverse Kinematics
Use Law of Cosines to find α cos α = (a2 + d2 – b2)/2ad cos α = (a2 + x2 + y2 – b2)/2ad (0,0) α b (x,y) θ a d
SLIDE 15
Simple Inverse Kinematics
Use arctangent to find β then φ β = atan2(y,x) φ = α – β (0,0) φ a b (x,y) θ β
SLIDE 16 Simple Inverse Kinematics
- Only works for single joint
- Always planar because only three points
- Works great for elbows, knees, etc.
(0,0) φ a b (x,y) θ
