: research

Visualizing parameters of human motion

Summer 1999
With Nancy Pollard

Computer Graphics Lab
Brown University



Background

Realistic animation of human motion has a wide variety of potential applications, ranging from entertainment to sports training and medicine.  Current techniques for animating human characters focus on the motion itself and typically output a rendering of the moving character. Many applications, however, especially those in the sciences and sports, could benefit from the visual display of supplementary information about the motion. For example, a visual display of weight distribution and active muscle groups could help a dancer to understand not just what a motion looks like, but how it is performed. Physically based approaches to animation such as that pursued by both Pollard and Hodgins make this type of supplementary information readily available. For example, we have developed techniques to scale simulated motions such as running and cycling to new individuals. Ground contact forces and joint torques are an intrinsic part of the simulated motions, and a user comparing the performance of different individuals would wish to compare these quantities across simulations. This information, however, is not currently provided to the user in an intuitive form.

We are using a Java3D environment, modeled after an in-house non-photorealistic rendering system developed at Brown (Markosian, Siggraph 1997). This system is now in use by a substantial number of graduate and undergraduate students at Brown, and the Java3D implementation combines the system's intuitive camera manipulation with the the latest Java3D benefits.

The physical parameters to be displayed in this project come directly from physically based simulations or from other sources such as motion capture data. When other motion sources are used, we will calculate the required information using techniques such as inverse dynamics, a method used in robotics to calculate the control torques needed to generate a desired motion.

This project is in conjunction with ongoing research at Georgia Tech.  Currently, the Gatech group is exploring new techniques for emphasizing the overall dynamics of human motion. 

Research (Summer 1999)

The first few weeks of this project were spent studying existing research on human animation, information design, and biomechanics.  Simplicity and elegance in motion visualization are crucial, and so we researched the techniques of Marey, Muybridge, and Tufte, scientists and artists who have analyzed motion.  Their work inspired certain design guidelines for this project:  clarity of presentation, visual honesty, and refined use of color.  Also during this time we ported existing motion playback code from C++ to Java3D, and included a Java trackball camera designed by other Brown students.

Currently, we have designed three devices to represent different aspects of a runner's motion, and have more in progress.  These devices include a center-of-mass shadow, ground contact force arrows, and joint trajectory tracers.  Each can be used on either one or multiple runners, for teaching or comparison purposes.
 


  Center of Mass Shadow
Center of Mass Shadows map the location of the runner's gravitational center onto the ground plane.  The Shadows' sizes change based on the distance between the center of mass and the ground.
A center of mass (COM) shadow is the projection onto the ground plane of a runner's center of mass.  The circular shadow has radius proportional to the COM's height (z direction).  As the runner's body shifts weight in the x-y plane, the shadow moves across the ground.  When the COM's z coordinate decreases (i.e., the COM moves closer to the ground), the shadow's radius increases linearly.  Similarly, when the COM moves away from the ground, the shadow's radius shrinks.  Using motion tracks from the simulator developed by Hodgins and Pollard, we found that both an adult male model and a child model project their COMs on the area between their feet.
 
  Ground Contact Force Arrows
Ground Contact Force Arrows change in length and rotation based on the magnitude and direction of the force applied by the ground on the runner's foot.

Comparison between original child and machine-scaled child (MPEG) (QT)
Ground Contact Force Arrows display the magnitude and direction of the forces applied to runners' bodies as they hit the ground.  The simulator generates force data for support-phase frames in the form of a reference point and force vector.  The Force Arrows disappear for non support-phase frames, since no ground contact forces are applied then.  We have found that at the end of the flight phase, the foot strikes the ground with a significant forward force.  A Force Arrow shows the equal-but-opposite force applied by the ground on the foot.  Then, as the foot stabilizes and prepares for lift-off, the ForceArrow changes in length and rotation, intuitively showing the effects of the runner's shifting weight.
 

Joint Trajectory Tracers
Joint Trajectory Tracers graph the path of a joint across time.  Here, the runner's right elbow follows a periodic path with low amplitude.
Tracers plot the movement of a joint over time.  Every given number of frames, a Tracer appears in the Java3D universe, marking the location of the joint at that frame.  Tracers  emphasize the periodicity of a motion, as well as highlight any differences between cycles.  Tracers also enhance motion comparisons, displaying, for example, that at a particular frame, Runner A's elbow is much higher than Runner B's elbow.

Future Ideas

Related Reading

Information Design Biomechanics

Related Projects

 

Brown Researchers

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Other research projects