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Animation. Low-Level behaviors Overview Keyframing Motion Capture Simulation. Low-Level Behaviors. Keyframing Motion Capture Simulation. Generating Motion. What matters? Quality of motion appropriate for rendering style and frame rate Controllable from UI Controllable from AI
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Animation Low-Level behaviors Overview Keyframing Motion Capture Simulation Game Design
Low-Level Behaviors • Keyframing • Motion Capture • Simulation Game Design
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 Game Design
Keyframe Example Game Design
Keyframing • Fine level of control • Quality of motion depends on skill of animator Game Design
Motion Capture • Natural-looking motion • Hard to generalize motions • Registration is difficult • “Weightless” according to professional animators Game Design
Motion CaptureImages courtesy Microsoft Motion Capture Group Game Design
Simulation (Broadly Defined) • Physics is hard to simulate • Pseudo-physics is somewhat hard • Control is very hard • Gives Generalization + Interactivity Desired Behavior Forces and Torques Model Numerical Integrator User/AI Control State Graphics Game Design
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 Game Design
Keyframing Game Design
Keyframing Game Design
Keyframe Example Game Design
Keyframing • Fine level of control • Quality of motion depends on skill of animator Game Design
Hand Drawn Animation -- 2D • Sketches • Pencil tests • Inking • Coloring • Digitize to sprites Game Design
Computer Animation: 2D or 3D • Sketches • Models and materials • Key configurations • Playback of motion or render to sprites Game Design
The development process: Adjust trajectory Playback motion Parameters: Locations Joint angles Shape -- flexible objects Material properties Camera Motion Lighting Keyframing Game Design
Keyframing Interpolation • Inbetweening v 1,2,3 4 5 6 7,8,9… Linear time v 1,2,3 4 5 6 7,8,9… Slow in, Slow out time Game Design
Interpolation Example Game Design
Key Frames 1 2 3 Game Design
Key Frames • Timeline • 1 2 3 3 1 Game Design
Inbetweening • Frame dependent (“wrong”) slow-in/out • Iterate once per frame • This is a variant on the Infinite Impulse Response (IIR) filter: Game Design
Inbetweening • Frame-Independent (“right”) slow in/out • Compute acceleration a Time of last frame Time Tend Game Design
Spline-driven Animation y x,y = Q(u) for u:[0,1] • Equal arc lengths • Equal spacing in u x Game Design
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) Game Design
Keyframing -- Constraints • Joint limits • Position limits • Inverse kinematics Game Design
Keyframing -- Constraints Game Design
Coordinate Systems Game Design
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 Game Design
Inverse Kinematics Game Design
Inverse Kinematics Game Design
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 Game Design
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) Game Design
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 Game Design
Motion Capture • Goals: • Realistic motion • Lots of different motions (300-1000) • Contact • Appropriate game genres • Sports • Fighting • Human characters Game Design
Applications Movies, TV Video games Performance animation Game Design
Plan out Shoots Carefully • Know needed actions (80-100 takes/day) • Bridges between actions • Speed of actions • Starting/ending positions • Hire the right actor • Watch for idiosyncrasies in motion • Good match in proportions Game Design
Sensor Placement • Place markers carefully • Capture enough information • Watch for marker movement • Check data part way through shoot • Videotape everything! Game Design
Technology • Numerous technologies • Record energy transfer • Light • Electromagnetism • Mechanical skeletons Game Design
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 Game Design
Technology • Passive reflection --Acclaim, Motion Analysis • Automatically digitized • 240Hz • Not real-time, Correspondence • 3+ markers/body part • 2+ cameras for 3D position data Game Design
Technology • Vicon Motion Systems • Retroreflective paint on reflectors • Lights on camera • Very high contrast markers Game Design
Technology • Active light sources -- Optotrak • Automatically digitized • 256 markers • 3500 marker/sec • Real-time • Specialized cameras Game Design
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 • 6DOF Realtime • 30-144 Hz 13-18 markers Game Design
Technology • Exoskeleton + angle sensors • Analogous • Tethered • No identification problem • Realtime - 500Hz • No range limit - Fit • Rigid body approximation Game Design
Dataglove Low accuracy Focused resolution Monkey High accuracy High data rate Not realistic motion No paid actor Technology Mechanical motion capture Game Design
Technology • Technology issues • Resolution/range of motion • Calibration • Accuracy • Occlusion/Correspondence Game Design
Animation Issues • Style • Scaling • Generalization Game Design