Animating reactive motion using

momentum-based inverse kinematics

CASA 2005

Animating reactive motion using

momentum-based inverse kinematics

Taku Komura S.L. Ho W.H. Lau

Department of Computer ScienceCity University of Hong Kong

Outline

○Introduction

○Generating Reactive Motions

○Results and Discussion

Introduction

○ Motion capture data has become a popular and an effective means of animating human figures.• Due to the quality and realism of the result.

○ The synthesis of motion capture data is a challenging research problem.• Only a limited number of methods consider

reactive motions.

Introduction

○ Interactive generation of reactive motions are very important to many applications.• Computer games

○ A new method to generate reactive motions for arbitrary external perturbations• Pushing, pulling, or hitting vs.

standing, walking, or running

Motion Database

○ Basic standing, walking, and running• Primary motion

○ Pushed/pulled from eight different directions• Front, side, front-side, and back-side

Outline

○Introduction

○Generating Reactive Motions

○Results and Discussion

Generating Reactive Motions

○ Step 1: Adding impact• The user specifies the position, amplitude

and direction.

○ Step 2: Motion search• The one minimized the criteria is chosen.

○ Step 3: Editing found motion• According to the strength of the perturbation

○ Step 4: Generating final motion• Momentum-based IK

Step 1: Adding impact

○ The amount of impact, is calculated by subtracting the linear momentum of the body before the impact from that after the impact.

○

p: the position of the impactxg: the position of COM

L

LxpM g )(

Step 2: Motion Search

○ The reactive motion which includes a frame that minimizes the following criteria will be selected from the database.

○ It is much simpler than the one used by other graph-based motion stochastic methods.

rli

iii IIkvvk,

2102

20,1, ||||||||

Step 3: Editing found motion

○ The positions of the foot steps must be changed according to the amplitude and direction of the impact.

○ The velocity of the body is linearly dependent on the displacement of the landing foot onto the ground.

AAstep

zozr vvmn

FFd

,,

Step 3: Editing found motion

○ Blends the angular momentum.

○ Let the difference at the moment blending starts be .

Iwtt

ttIwIw

il

i

101

Iw

Step 4: Generating final motion

○ Once the reactive motion is chosen and edited, this motion will be blended into the primary motion using momentum-based IK.

qJIw

qJxmL

tIwtLtxtIwtLtx

tIwtLtx

Iw

Lg

))(),(),(())(),(),()(1(

))(),(),((

111000

Outline

○Introduction

○Generating Reactive Motions

○Results and Discussion

Results

Discussion

○ Foot sliding can easily happen if the primary motion and the reactive motion is simply interpolated.

○ The momentum-based constraints help keep the motion realistic even after editing the motion or adding new constraints.

○ The criteria to select the motion in the database is much simpler than previous methods such as in Lee et al., the computation time is much shorter.

○ But for offline process, such as Motion Graph, there is no doubt works better.

END