Graphics. Computer Animation 고려대학교 컴퓨터 그래픽스 연구실. kucg.korea.ac.kr 1

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Graphics

Graphics Lab @ Korea University Graphics Lab @ Korea University

kucg.korea.ac.kr 1

Computer Animation

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KUCG

Computer Animation



What is Animation?



Make objects change over time

according to scripted actions



What is Simulation?



Predict how objects change over

time according to physical laws

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Outline



Principles of Animation



Keyframe Animation

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Principle of Traditional

Animation – Disney –



Squash and Stretch



Slow In and Out



Anticipation



Exaggeration



Follow Through and Overlapping Action



Timing



Staging



Straight Ahead Action and Pose-to-Pose Action



Arcs



Secondary Action

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Squash and Stretch

Squash

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Computer Animation



Animation Pipeline



3D modeling



Motion specification



Motion simulation



Shading, lighting, & rendering



Postprocessing

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Outline



Keyframe Animation

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Keyframe Animation



Define Character Poses at Specific Time

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Keyframe Animation



Interpolate Variables Describing Keyframes

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Inbetweening



Linear Interpolation

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Inbetweening



Spline Interpolation

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Inbetweening



Maybe good enough

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Inbetweening



Maybe good enough

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Inbetweening

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Outline



Keyframe Animation

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Articulated Figures



Character Poses Described by Set of Rigid

Bodies Connected by “Joints”

Base

Arm

Hand

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Articulated Figures

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Articulated Figures



Joints Provide Handles for Moving

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Summary



Animation Requires ...



Modeling



Scripting



Inbetweening



Lighting, shading



Rendering



Image processing

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Motion Capture



Motion capture = Motion tracking = Mocap



Process of recording movement and translating

the movement onto a digital model



To animate digital character models in 3D animation



Records actions of human actors, and use the

information gathered

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Motion Capture



Motion of an actor can be captured with various

methods



Markers



Without Markers (Optical)



Mechanical motion

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Motion Capture



Application



Military training

simulations



Video games



Films



Sports



Medical applications

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Motion Capture



Advantages



Can replace key-frame based animation with

smoother animation with much less efforts



Recording various test animations can be performed

without much concern



Complex, and physically accurate animation

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Motion Capture



Disadvantages



Special hardware & software are required



Required software, hardware and personnel can be

expensive



Motions which are not physically correct cannot be or

are hard to be captured

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Motion Capture

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Motion Capture

The Lord of The Rings,

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Overview of Kinematics &

Dynamics



Kinematics



Consider only motion



Determined by positions, velocities, accelerations



Dynamics



Consider underlying forces

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Example: 2-Link Structure



Two Links Connected by Rotational Joints

“End-Effector”

X=(

x

,

y

)

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Forward Kinematics



Animator Specifies Joint Angles:

Q

1

and

Q

2



Computer Finds Positions of End-Effector: X

X=(

x

,

y

)

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Inverse Kinematics



What If Animator Knows Position of

“End-Effector”



Animator Specifies End-Effector Positions: X



Computer Finds Joint Angles:

Q

1

and

Q

2

“End-Effector”

X=(

x

,

y

)

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Inverse Kinematics



Problem for More Complex Structures



System of equations is usually under-defined



Multiple solutions

X=(

x

,

y

)

(0, 0)

Three unknowns:

Q

1

, Q

2

, Q

3

Two equations:

x

,

y

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Summary



Forward Kinematics



Specify conditions (joint angles)



Compute positions of end-effectors



Inverse Kinematics



“Goal-directed” motion



Specify goal positions of end effectors



Compute conditions required to achieve goals

Inverse kinematics provides easier

specification for many animation tasks,

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Overview



Kinematics



Consider only motion



Determined by positions, velocities, accelerations



Dynamics



Consider underlying forces

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Dynamics



Simulation of Physics Insures Realism of

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Space Time Constraints



Animator Specifies Constraints



What the character’s physical structure is



e.g., articulated figure



What the character has to do



e.g., jump from here to there within time

t



What other physical structures are present



e.g., floor to push off and land



How the motion should be performed



e.g., minimize energy

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Advantages



Free animator from having to specify details of

physically realistic motion with spline curves



Easy to vary motions due to new parameters and/or

new constraints



Challenges



Specifying constraints and objective functions

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

Adapting Motion

Original Jump

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Adapting Motion

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Adapting Motion

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Editing Motion

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Example

http://www.naturalmotion.com/endorphin.htm

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Space Time Constraints



Advantages



Free animator from having to specify details of

physically realistic motion with spline curves



Easy to vary motions due to new parameters and/or

new constraints



Challenges



Specifying constraints and objective functions

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Physical simulation



Simulate natural phenomena based upon the

law of physics



Predict how objects change over time according to

physical laws



Physical simulations



Rigid, Soft bodies



Cloth, hair



Fluids (Liquids, Gases)



etc

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Example



“Fracturing Rigid Materials”, IEEE TVCG 2007,

Bao et al.

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Example



“Controlling fluid animation with Geometric

Potential”, CASA 2004, Hong and Kim

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Example



"Wrinkled Flames and Cellular Patterns",

SIGGRAPH 2007, Hong et al.

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Rigid, Soft bodies



Zhaosheng Bao, Jeong-Mo Hong, Joseph Teran and Ron Fedkiw, "Fracturing Rigid Materials",

IEEE Transactions on Visualization and Computer Graphics, Volume 13, Issue 2, pp.370-378,

2007.



Shinar, T., Schroeder, C. and Fedkiw, R., "Two-way Coupling of Rigid and Deformable Bodies",

ACM SIGGRAPH/Eurographics Symposium on Computer Animation (SCA), edited by Doug

James and Markus Gross, pp. 95-103 (2008).

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Cloth, Hair



Selle. A, Su, J., Irving, G. and Fedkiw, R., "Highly Detailed Folds and

Wrinkles for Cloth", IEEE TVCG (in press).



Selle, A., Lentine, M. and Fedkiw, R., "A Mass Spring Model for Hair

Simulation", SIGGRAPH 2008, ACM TOG 27, 64.1-64.11 (2008).

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Fluid simulation

(Water, Smoke)



Losasso, F., Talton, J., Kwatra, N. and Fedkiw, R., "Two-way Coupled SPH and Particle Level

Set Fluid Simulation", IEEE TVCG 14, 797-804 (2008).



T. Kim, Nils Thürey, Doug James, and Markus Gross, “Wavelet turbulence for fluid simulation”,

SIGGRAPH 2008, ACM TOG 27.

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Physical simulation



Why do we need physical simulation



It is nearly impossible to produce natural physical

phenomena manually without extreme effort



Can generate physically accurate motions



Can generate huge scale animations such as

explosion, tornado, tidal wave



Actors do not have to risk possible dangers of

extreme environments

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Applications of Physical

Simulation

Movie

“

Pirates of the Caribbean(2007)”

Evian TV CF.

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Summary



Kinematics



Forward kinematics



Animator specifies joints (hard)



Compute end-effectors (easy)



Inverse kinematics



Animator specifies end-effectors (easier)



Solve for joints (harder)



Dynamics



Space-time constraints



Animator specifies structures & constraints (easiest)



Solve for motion (hardest)

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Navier-Stokes Equations

Appendix : Fluid Simulation

Advection

External

force

Diffusion

Projection

Reference:

Jos Stam, "Stable Fluids", In

SIGGRAPH 99 Conference

Proceedings, Annual Conference Series

, August 1999, 121-128.

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The advection step moves the density through a static velocity field.

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The effect of

Density Diffusion

Velocity Diffusion

makes the fluid move more like a rigid object.

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To obtain an incompressible field we simply subtract

the gradient field from our current velocities.