Animating Human Athletes

By J.K. Hodgkins and

W.L. Wooten

Arjun Rihan

CS 99K: Digital Actors

Introduction People are skilled at perceiving the

subtle details of human motion If synthesized human motion is to

be compelling, it is necessary to create actors for computer animations and virtual environments that are realistic

Algorithmic Approach Control algorithms that allow a

rigid-body human model to run, vault and ride a bicycle in various environments

Built from a common toolbox: state machines, inverse kinematics, etc.

Advantages Can be easily modified to compute

similar but different motions Greater interaction possible,

especially in virtual environments Possible to generate secondary or

composite motion

Disadvantages

Difficult to design Range of parameter variation

allowed is relatively narrow Workstation speed limits their use.

For example, the simulations show earlier run from 6 to 20 times slower than real time in virtual environments

Examples

QuickTime™ and aCompact Video decompressorare needed to see this picture.

Compare the actor to the human diver

Example

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Notice the how the actor mimics reality by trying to regain balance after the jump

Dynamic Behaviors

Each simulation contains: - the equations of motion for model

and environment - control algorithms for balancing,

running, etc. - graphical user interface for simple

top-level parameter control

Dynamic Behaviors (contd.)

In the case of a gymnast performing a vault, the control system obtains the following parameters and then computes the required forces and positioning of the limbs from control algorithms

The Human Models

Rigid links connected by rotary joints with varying degrees of freedom

Examples of possible constraints on the degrees of freedom of the foot

Modeling Phases with State Machines The distinct phases and corresponding

changes in control actions are generated by a state machine

This is a schematic of the state machine for the runner

Transitions refer to the active leg

Active leg and idle leg keep alternating

Another State Machine - Gymnast

The state machine determines the control laws that are in effect for each phase of the vault

What This Means The algorithms presented here

enable an animator to generate motion for several dynamic behaviors

Evaluation

Through side-by-side comparison of video footage and animation, results appear to be good (What do you think?)

Comparison with biomechanical data is favorable

Evaluation (contd.)

A final form of evaluation would be a Turing test

Would involve direct comparison between simulated and human data on the same graphical model

Which motion would viewers prefer, given that that they wanted a more “natural” motion?

Next Steps

How can we make it easier to generate control algorithms for a new behavior? By using a toolbox of general techniques to

construct them as demonstrated here How can we make the motion more natural?

Combine this approach with motion-capture and keyframing

Fine-tune the algorithms with human aesthetics

More Examples

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