Body Kinematics

Motion Capture

Joints, Bowel

Kinematics

Kinematics motion of the body with respect to time

• Position
• velocity
• acceleration

Skeleton

skeleton=tree like data structures representing a hierarchy of joints

joint hierarchy

Joints

Joints: local rotating frame of reference

$$F_j^i=\begin{bmatrix} F_j^i&d_j^i\\ 0 & 1 \end{bmatrix}$$

where

• i=joint index
• i=Parent of joint j
• $d_j^i$=bone(link) between joint j and joint i

Each joint node contains data

• Angle/quaternions
• bone length
• Pointers(links) to parent joint
• Pointers(links) to child joints
• Local transform
• global transform
• other data
  • moment of in
  • Joint limits
  • presents (bind pose)

Each joint a Frame of ref

$F_j^i\Rightarrow$ local frame of ref

j is the parent of joint j

$F_j^0\Rightarrow$ global transform, now joint j is positioned and oriented w.s.t. world

To find the position and orientation of any joint in the skeleton wal the kinematic chain from woot to that joint

$F_3^0=F_1^0F_2^1F_3^2F_4^3$

position of joint 4 w.r.t world

$\vec p^0=F_4^0\vec p^4$, $\vec p^4=[0,0,0,1]^T$

Body state vector

$\vec x$ is an (m x 1) vector of end joint positions

$\vec\Theta$ is an (n x 1) body joint vector of the form,

$\Theta$ Combines positions and orientations of all the joints

$$\vec\theta_j=\begin{bmatrix} x\\y\\z\\\theta_x\\\theta_y\\\theta_z \end{bmatrix}$$

most general case for joint j

$$\vec \Theta=\begin{bmatrix} \vec\theta_1\space\vec\theta_2\space...\space\vec\theta_N \end{bmatrix}^T$$

6 dofs

$$\left . \begin{aligned} x_1 &= a_1 + b_1 \\ x_2 &= a_2 + b_2 \\ x_3 &= a_3 + b_3 \end{aligned} \right\} \quad \Rightarrow \quad x = a + b$$

want to know position of joint 3 in the world

$$\vec p_3^0=F_3^0\begin{bmatrix} 0\\0\\0\\1 \end{bmatrix}=H_T(\vec d_1^0)H_R(\theta_1)H_T(\vec d_2^1)H_R(\theta_2)H_T(\vec d_2^1)H_R(\theta_3)\begin{bmatrix} 0\\0\\0\\1 \end{bmatrix}$$

$F_3^0=F_1^0F_2^1F_3^2$

$$\vec p_3^0=\begin{bmatrix} l_1\cos\theta+l_2\cos(\theta_1+\theta_2)\\ l_1\sin\theta+l_2\sin(\theta_1+\theta_2) \end{bmatrix}+\vec p_1^0$$

$\vec x =f(\vec \Theta)=$ position of joints

$\vec v=\dot{\vec x}=$ velocity

$$\vec v=\frac{d\vec x}{dt}=\dot{\vec x}\\ \vec x=f(\vec\Theta)\\ \frac{d\vec x}{dt}=\frac{f}{d\vec\Theta}$$

$\dot {\vec x}=J\dot {\vec \Theta}$

$J=$ jacobian matrix

$\dot {\vec \Theta}=J^{-1}\dot {\vec x}$

Leg example

quiz：给一个chain，让求一个p的坐标

下次课讲如何计算任意一个chain的J

$\vec v=\vec w\times\vec r$

Shape animation

Forward Kinematic (FK)

Notations

$\vec x=f(\vec\Theta)$

$\vec p_4^0=F_4^0\vec p_4^4$

where $\vec\Theta$ a set of homo trans

position: $\vec x=f(\vec\Theta)$

velocity: $\vec v=\dot {\vec x}=\frac{d\vec x}{d t}=\frac{\part f}{\part\Theta}\frac{\vec\Theta}{dt}=J\dot{\vec\Theta}$

Angular velocity: $\vec \omega=\hat a\dot\phi$

$\vec v=\omega\times\vec r$

where $\hat a$=axis, $\dot\phi$=spin rate

Euler Angle rate: $\dot{\vec\theta}=\begin{bmatrix}\dot\theta_x\\\dot\theta_y\\\dot\theta_z\end{bmatrix}$

where $\vec\theta$

$\dot{\vec\theta}\ne\vec \omega$

only if axis is one of the x-y-z axis, they are the same

$\dot\theta\rightarrow\dot\omega$

Euler Angle rates to angular velocity conversion

$\vec \omega=\dot\phi\hat\alpha$

$\vec \omega^j=L_j(\theta_x,\theta_y,\theta_z)\dot{\vec\theta}$

$R_1=R_z R_yR_x$

$R_0^1=R_1^T=R_x^TR_y^TR_z^T$

$$L_j(\theta_x,\theta_y,\theta_z)=\begin{bmatrix} 1&0&-\sin\theta_y\\ 0&\cos\theta_x&\sin\theta_x\cos\theta_y\\ 0&-\sin\theta_x&\cos\theta_x\cos\theta_y \end{bmatrix}$$

Inverse Kinematic (IK)

Notations

$\vec\Theta=f^{-1}(\vec x)$

almost impossible to compute amony

Very difficult to compute, analytically

Ideally, $\dot{\vec\Theta}=J^{-1}\dot{\vec x}$

Integrate things

$\vec v=\omega\times\vec r+\dot{\vec r}$

Spin 1

if joint 2, 3 is fixed, joint 1 spin at $\omega_1^0$, then:

$\vec v_{14}=\vec \omega_1^0\times\vec r_4^1$

where $\vec r_4^1=\vec p_4^0-\vec p_1^0$

Spin 2

if joint 3 is fixed, joint 1 spin at $\omega_1^0$, then:

$\vec v_4=\vec \omega_1^0\times\vec r_4^1$

where $\vec r_4^1=\vec p_4^0-\vec p_1^0$

total:

$\vec v_4^0=\vec v_{14}^0+\vec v_{24}^0+\vec v_{34}^0+\vec v_1^0$

rewrite:

$$\begin{aligned} \vec v_{14}^0 &=\vec\omega_1^0\times\vec r_{14}^0\\ &=-\vec r_{14}^0\times\vec\omega_1^0\\ &=-\vec r_{14}^0\times R_1^0\vec\omega_1^1\\ &=-\vec r_{14}^0\times [\begin{array}{c:c}\hat a_x&\hat a_y&\hat a_z\end{array}]\vec\omega_1^1\\ &=B_1\vec{\omega}_1^1 \end{aligned}$$

where $R_1^0=[\begin{array}{c:c}\hat a_x&\hat a_y&\hat a_z\end{array}]$

$$\begin{align} \vec v_4^0 &=[\begin{array}{c:c}B_1&B_2&\dots &B_n\end{array}] \begin{bmatrix}\vec\omega_1^1\\\vec\omega_2^2\\\vec\omega_3^3\end{bmatrix}\\ &=[\begin{array}{c:c}B_1L_1&B_2L_2&\dots &B_nL_n\end{array}]\dot\Theta \end{align}$$

Euler Angle rates to angular velocity conversion

Jacobian Matrix (J) Calculation

N is the number of joint

$$\begin{align} J &=[\begin{array}{c:c}J_1&J_2&\dots &J_N\end{array}]\\ &=[\begin{array}{c:c}B_1L_1&B_2L_2&\dots &B_NL_N\end{array}] \end{align}$$

where

$$B_i=$$

Pseudo Inverse

IK want to compute $J^{-1}$, can only normally do this if J is square

given $\vec x$, desired velocity of the end joint

2 types of "pseudo" inverse

• left => more DOF in x

• right => more DOF in y

  让方阵是比较小的那个纬度

$\vec x=J\vec\Theta$

$\dim\vec x=m\times1$

$\dim\vec\Theta=n\times1$

$\dim J=m\times n$

left pseudo

When: m>n

$\dot{\vec x_d}=J\dot{\vec\Theta}$

$J^T\dot{\vec x_d}=J^TJ\dot{\vec\Theta}$

$\dot{\vec\Theta}=(J^TJ)^{-1}J^T\dot{\vec x_d}$

$J^\dagger=(J^TJ)^{-1}J^T$

right pseudo

most likely

When: m<n

$\dot{\vec x_d}=J\dot{\vec\Theta}$

choose $\dot\Theta=J^T\vec a$

$$\dot{\vec x_d}=JJ^T\vec a\\ (JJ^T)^{-1}\dot{\vec x_d}=\vec a\\ \dot{\vec\Theta}=J^T(JJ^T)^{-1}\dot{\vec x_d}$$

$J^T\dot{\vec x_d}=J^TJ\dot{\vec\Theta}$

$\dot{\vec\Theta}=J^T(JJ^T)^{-1}\dot{\vec x_d}$

choose $\vec b=\dot{\vec x_d}=\vec v_d$

$\dot{\vec x}=JJ^T(JJ^T)^{-1}b$

$J^\dagger=J^T(JJ^T)^{-1}$

---

Discrete time, k = frame#

$\dot{\vec x_d}(t_k)=\frac{\vec x_d(t_k)-\vec x(t_{k-1})}{\Delta t}$

$\dot{\vec \Theta}(t_k)=\frac{\vec \Theta(t_k)-\vec \Theta(t_{k-1})}{\Delta t}$

Backward differences

$$\frac{\vec \Theta(t_k)-\vec \Theta(t_{k-1})}{\Delta t}=J^{-1}\frac{\vec x_d(t_k)-\vec x(t_{k-1})}{\Delta t}\\ \vec \Theta(t_k)=\vec \Theta(t_{k-1})+J^{-1}(\vec x_d(t_k)-\vec x(t_{k-1}))$$

Leg example:

Know $\vec p_5^0(t_0)$

Want $\vec p_5^0\Rightarrow\vec p_d^0$

What is $\vec x_d(t)$

$\vec x_d(t)=Lerp(\vec p_5^0(u), \vec p_d^0(u),u)$

---

$\dot{\vec\Theta}$ Joint velocity spance, dim n

$\dot{\vec x}$ end Joint velocity spance, dim m

$\dot{\vec x}=J\dot{\vec\Theta}$

often interested in values of $\dot{\vec\Theta}$ that produce $\dot{\vec x}=0$

trivial case: $\dot{\vec\Theta}=0$

When $n>m$, it is possible that $0=J\Theta$ When $\dot{\vec \Theta}\neq 0$

null space

IK solver

$\Theta_{null}=J^T(JJ^T)^{-1}\vec b-\vec c$

$x=J\Theta=JJ^T(JJ^T)^{-1}\vec b-J\vec c=0$

choose $b=J\vec c$, null space

$\dot{\vec{\Theta}}_{null}=[J^T(JJ^T)^{-1}J\vec c-\vec c]=[J^T(JJ^T)^{-1}J-I]\vec c=[J^{-1}J-I]\vec c$

c is arbitrary vector at dim $n\times1$ same dim as $\theta$

spring-like behavior for joints

$\vec \Theta_d=$desired joint angles

$\vec \Theta=$ actual joint angles

$\dot{\vec\Theta}=k({\vec\Theta_d}-\vec\Theta)$

IK solver

$\dot{\vec\Theta}=J^{-1}\dot{\vec x}_d+\dot{\vec\Theta}_{null}$

$x=J\dot{\vec\Theta}=J(J^{-1}\dot{\vec x}_d+\dot{\vec\Theta}_{null})$

Choose $\dot{\vec \Theta}_c=k(td-t)$

$\dot{\vec \Theta}=J^{-1}\dot{\vec x}_d+[J^{-1}J-I]k(\vec\Theta_d-\vec\Theta)$

primary constraint, secondary constraint

---

some times there are problems with inversion of J, when loss of 2 degree of freedom

$\dagger$

In this case use a "damped pseudo inverse"

Normally $J^\dagger=J^T(JJ^T)^{-1}$

Damped $J^\dagger=J^T(JJ^T+\lambda I)^{-1}$

with $\lambda<<1$, I is identity matrix

Limb-Based Approach

Main idea

• law of Cosine

• dot product

• cross product

Joint 3 orientation

$\dot{\vec x_d}=\frac{\vec x_d(t_k)-\vec x(t_k)}{\Delta t}$

Steps

1. Compute only $\theta_2$ to get distance between joint 1 and joint 3 equal to $\|r_d\|$

   $\phi=\arccos\frac{(l_1)^2+(l_2)^2-(r_d)^2}{2\|l_1\|\|l_2\|}$

   $\theta_2=\pi-\phi$

2. Compute quaternion $q_1$ at joint 1 to rotate $r_d$ so it points at $p_d$

   $\Delta\theta_1=\arccos\frac{\vec d\cdot \vec r_d}{\|\vec d\|\|\vec r_d\|}$

3. Joint 3 Orientation (optional)

   if indicate

Coordinate Cyclic Descent (CCD)

Main Concept: If rotate joint 1 by angle get end joint 2 as close as possible to pd

motion capture

Create a kinematic chain of joints of length n ( j = 1 to n )

1. Start with the second to last joint in the chain (i.e. j = n-1)
2. Compute error between end joint ( $p_n$ ) and goal ( $p_d$ )
3. Compute $r_{jn}$ and $r_{dj}$ for joint $j$
4. Compute $\Delta \theta_j$ and $\hat a_j^j$ for joint $j$
5. Scale joint rotation $\Delta \theta_j=c\Delta\theta_j$
6. Compute $q_j=q_j\cdot\Delta q_j$
7. Update $p_n$ after setting joint j rotation to $q_j$
8. Update $j=j-1$