Oct 3, Fall 2005 Game Design 1 Animation Low-Level behaviors Overview Keyframing Motion Capture Simulation

Oct 3, Fall 2005Game Design2 Low-Level Behaviors  Keyframing  Motion Capture  Simulation

Oct 3, Fall 2005Game Design3 Generating Motion  What matters? –Quality of motion appropriate for rendering style and frame rate –Controllable from UI –Controllable from AI –Personality of the animated character

Oct 3, Fall 2005Game Design4 Keyframe Example

Oct 3, Fall 2005Game Design5 Keyframing  Fine level of control  Quality of motion depends on skill of animator

Oct 3, Fall 2005Game Design6 Motion Capture  Natural-looking motion  Hard to generalize motions –Registration is difficult –“Weightless” according to professional animators

Oct 3, Fall 2005Game Design7 Motion Capture Images courtesy Microsoft Motion Capture Group

Oct 3, Fall 2005Game Design8 Simulation (Broadly Defined)  Physics is hard to simulate  Pseudo-physics is somewhat hard  Control is very hard  Gives Generalization + Interactivity User/ AI Desired Behavior Control Forces and Torques Model Numerical Integrator Graphics State

Oct 3, Fall 2005Game Design9 When to Use What Method?  Keyframing –Sprites and other simple animations –Non-human characters –Coarse collision detection  Motion Capture –Human figures –Subtle motions, long motions  Simulation –Passive simulations –When interactivity w/ motion is important

Oct 3, Fall 2005Game Design10 Keyframing

Oct 3, Fall 2005Game Design11 Keyframing

Oct 3, Fall 2005Game Design12 Keyframe Example

Oct 3, Fall 2005Game Design13 Keyframing  Fine level of control  Quality of motion depends on skill of animator

Oct 3, Fall 2005Game Design14 Hand Drawn Animation -- 2D  Sketches  Pencil tests  Inking  Coloring  Digitize to sprites

Oct 3, Fall 2005Game Design15 Computer Animation: 2D or 3D  Sketches  Models and materials  Key configurations  Playback of motion or render to sprites

Oct 3, Fall 2005Game Design16 Keyframing  The development process: Adjust trajectory Playback motion  Parameters: –Locations –Joint angles –Shape -- flexible objects –Material properties –Camera Motion –Lighting

Oct 3, Fall 2005Game Design17 Keyframing Interpolation  Inbetweening 1,2, ,8,9… Linear v 1,2, ,8,9… Slow in, Slow out v time

Oct 3, Fall 2005Game Design18 Interpolation Example

Oct 3, Fall 2005Game Design19 Key Frames

Oct 3, Fall 2005Game Design20 Key Frames  Timeline 

Oct 3, Fall 2005Game Design21 Inbetweening  Frame dependent (“wrong”) slow-in/out –Iterate once per frame –This is a variant on the Infinite Impulse Response (IIR) filter:

Oct 3, Fall 2005Game Design22 Inbetweening  Frame-Independent (“right”) slow in/out –Compute acceleration a TimeT end Time of last frame

Oct 3, Fall 2005Game Design23 Spline-driven Animation x x,y = Q(u) for u:[0,1]  Equal arc lengths  Equal spacing in u y

Oct 3, Fall 2005Game Design24 Reparameterize Arc Length  S= A(u) = arc length  Reparam:  Find: Bisection search for a value of u where A(u) = S with a numerical evaluation of A(u) (Details in Watt & Watt)

Oct 3, Fall 2005Game Design25 Keyframing -- Constraints  Joint limits  Position limits  Inverse kinematics

Oct 3, Fall 2005Game Design26 Keyframing -- Constraints

Oct 3, Fall 2005Game Design27 Coordinate Systems

Oct 3, Fall 2005Game Design28 Kinematics  The study of motion without regard to the forces that cause it Draw graphics Specify fewer Degrees Of Freedom (DOF) More intuitive control of DOF Pull on hand Glue feet to ground

Oct 3, Fall 2005Game Design29 Inverse Kinematics

Oct 3, Fall 2005Game Design30 Inverse Kinematics

Oct 3, Fall 2005Game Design31 What makes IK Hard?  Many DOF -- non-linear transcendental equations  Redundancies –Choose a solution that is “closest” to the current configuration –Move outermost links the most –Energy minimization –Minimum time

Oct 3, Fall 2005Game Design32 IK Difficulties  Singularities –Equations are ill-conditioned near singularities –High state-space velocities for low Cartesian velocities  Goal of “Natural Looking” motion –Minimize jerk (3rd derivative)

Oct 3, Fall 2005Game Design33 Motion Capture  What do we need to know? –X, Y, Z –Roll, Pitch, Yaw  Errors cause –Joints to come apart –Links grow/shrink –Bad contact points  Sampling Rate and Accuracy

Oct 3, Fall 2005Game Design34 Motion Capture  Goals: –Realistic motion –Lots of different motions ( ) –Contact  Appropriate game genres –Sports –Fighting –Human characters

Oct 3, Fall 2005Game Design35 Applications Movies, TV Video games Performance animation

Oct 3, Fall 2005Game Design36

Oct 3, Fall 2005Game Design37

Oct 3, Fall 2005Game Design38 Plan out Shoots Carefully  Know needed actions ( takes/day) –Bridges between actions –Speed of actions –Starting/ending positions  Hire the right actor –Watch for idiosyncrasies in motion –Good match in proportions

Oct 3, Fall 2005Game Design39 Sensor Placement  Place markers carefully –Capture enough information –Watch for marker movement  Check data part way through shoot  Videotape everything!

Oct 3, Fall 2005Game Design40

Oct 3, Fall 2005Game Design41 Technology  Numerous technologies  Record energy transfer –Light –Electromagnetism –Mechanical skeletons

Oct 3, Fall 2005Game Design42 Technology  Passive reflection – Peak Performance Tech –Hand or semi-automatically digitized –Video –Time consuming  Issues –No glossy or reflective materials –Tight clothing –Marker occlusion by props +High frames/sec

Oct 3, Fall 2005Game Design43 Technology  Passive reflection --Acclaim, Motion Analysis –Automatically digitized –240Hz –Not real-time, Correspondence –3+ markers/body part –2+ cameras for 3D position data

Oct 3, Fall 2005Game Design44 Technology  Vicon Motion Systems –Retroreflective paint on reflectors –Lights on camera –Very high contrast markers

Oct 3, Fall 2005Game Design45 Technology  Active light sources -- Optotrak –Automatically digitized –256 markers –3500 marker/sec –Real-time –Specialized cameras

Oct 3, Fall 2005Game Design46 Technology  Electromagnetic Transducers –Ascension Flock of Birds, etc –Polhemus Fastrak, etc  Limited range/resolution –Tethered (cables to box) –Metal in environment (treadmill, Rebar!) –No identification problem –6DOFRealtime – Hz13-18 markers

Oct 3, Fall 2005Game Design47 Technology  Exoskeleton + angle sensors –Analogous –Tethered –No identification problem –Realtime- 500Hz –No range limit- Fit –Rigid body approximation

Oct 3, Fall 2005Game Design48 Technology  Dataglove –Low accuracy –Focused resolution  Monkey –High accuracy –High data rate –Not realistic motion –No paid actor Mechanical motion capture

Oct 3, Fall 2005Game Design49 Technology  Technology issues –Resolution/range of motion –Calibration –Accuracy –Occlusion/Correspondence

Oct 3, Fall 2005Game Design50 Animation Issues  Style  Scaling  Generalization

Oct 3, Fall 2005Game Design51 Resolution  Positioning of camera

Oct 3, Fall 2005Game Design52 Markers, Calibration  Marker Placement –Location should move rigidly with joint –Stay away from bulging muscles, loose skin –Shoulders: Skeletal motion not closely tied to skin motion  Calibration –Zero position –Fine calibration by hand

Oct 3, Fall 2005Game Design53 Calibration  Finding Joint Locations –Move markers to joint centers Assume rigid links, rotary joints  Shoulder?

Oct 3, Fall 2005Game Design54 Calibration  Extract best limb lengths  Use estimator to compute limb length  Minimize or reject outliers

Oct 3, Fall 2005Game Design55 Calibration  Example estimator: –508 frames of walking –6 bad frames –Collarbone to shoulder: Hand editing: 13.3cm Estimator: 13.2cm Arithmetic mean: 14.1cm

Oct 3, Fall 2005Game Design56 Accuracy  Marker movement  Noise in sensor readings  Skew in measurement time  Environment restrictions  Frame rate –High frame rate allows good filtering

Oct 3, Fall 2005Game Design57 Camera Calibration  Internal camera parameters –Optical distortion of lens  External parameters –Position and orientation  Correlation between multiple cameras

Oct 3, Fall 2005Game Design58 Model-Based Techniques  Restricted search space for markers  Dynamics (velocity integration) –No infinite accelerations  Model of behavior  Model of bodies of occlusion –Objects don’t pass through each other

Oct 3, Fall 2005Game Design59 Scaling Animation  Contact  Movement style  Inverse kinematics

Oct 3, Fall 2005Game Design60 Generalizating Animation  Interpolation Synthesis for Articulated Figure Motion  Wiley and Hahn  IEEE CG&A v17#6

Oct 3, Fall 2005Game Design61 Generalizating Animation  Keyframes as constraints in a smooth deformation –Create functions by hand that warp the joint angle curves through time  Keyframe placing the ball on the racket at impact Motion Warping Witkin and Popovic, SIGGRAPH’95

Oct 3, Fall 2005Game Design62 Generalizating Animation  Motion Editing With Spacetime Constraints –Michael Gleicher –1997 Symposium on Interactive 3D Graphics

Oct 3, Fall 2005Game Design63 Blending Animations  Efficient Generation of Motion Transitions Using Spacetime Constraints –Rose, Guenter, Bodenheimer, Cohen –Siggraph ’96 –Uses dynamics to compute plausible paths –Blends these paths

Oct 3, Fall 2005Game Design64 Simulation  Modeling the real world with simple physics –Realism –A set of rules –Better interactivity  Objects or Characters

Oct 3, Fall 2005Game Design65 Passive -- No muscles or motors Active -- Internal source of energy

Oct 3, Fall 2005Game Design66 Equations of Motion  Water  Explosions  Rigid body models

Oct 3, Fall 2005Game Design67 Control Systems  Wide variety of behaviors  Transitions between behaviors  Controllable by AI or UI  Robust

Oct 3, Fall 2005Game Design68 Equations of Motion

Oct 3, Fall 2005Game Design69 Generating Motion  What matters? –Quality of motion appropriate for rendering style and frame rate –Controllable from UI –Controllable from AI –Skills of the animated character –Personality of the animated character

Oct 3, Fall 2005Game Design70 Keyframing  Fine level of control  Quality of motion depends on skill of animator

Oct 3, Fall 2005Game Design71 Motion Capture  Natural-looking motion  Hard to generalize motions –Registration is difficult  Often seems “weightless” – Bill Kroyer, Rhythm & Hues

Oct 3, Fall 2005Game Design72 Simulation (Broadly Defined)  Physics is hard to simulate  Pseudo-physics is somewhat hard  Control is very hard  Gives Generalization + Interactivity User/ AI Desired Behavior Control Forces and Torques Model Numerical Integrator Graphics State

Oct 3, Fall 2005Game Design73 When to Use What Method?  Keyframing –Sprites and other simple animations –Non-human characters –Coarse collision detection  Motion Capture –Human figures –Subtle motions, long motions  Simulation –Passive simulations –When interactivity w/ motion is important

Oct 3, Fall 2005Game Design74 Integration of Technologies  Layering –Add hand/finger motion later –Facial animation  Use keyframing to modify data –Fix holes in data  Use motion capture to drive simulation