The Perception of Walking Speed in a Virtual Environment

1. The Perception of Walking Speed in a Virtual Environment By T. Banton, J. Stefanucci, F. Durgin, A. Fass, and D. Proffitt Presentation by Ben Cummings For Animation (CS551-4), Fall 2003, with Dave Brogan

2. Perception of speed • Important for simulation of motion in virtual environments (driving or flight sims, for example) • Influenced by many cues, though we'll deal primarilly with visual and proprioceptual (physical sensation of body movement)

3. Radial Optical Flow • Results from looking down the axis of movement • Things expand from the vanishing point faster when they're closer to the viewer

4. Lamellar Optical Flow • Results from looking closer to perpendicular to the axis of motion • Objects' speed in image space stay constant when perpendicular

5. Optical Flow • When looking forward and moving, radial optical flow occurs towards the center of the view and lamellar flow occurs towards the periphery

6. Perception of movement speed • In reality, under normal conditions, people are usually good at judging their speed of movement • In virtual environments, there is markedly different performance • Particularly, in situations on treadmills with simulated correct optical flow, subjects perceive the speed of the visuals to be slower than their own speed

7. Perception of movement speed • The claim: The difference in actual motion and perceived motion is due to decreased lamellar flow in virtual environments • Display technology often cuts off the peripheral vision where lamellar flow is seen • Head mounted displays (HMDs) are usually about one third of the natural field of vision • Wall-mounted screens also usually occupy a small fraction of the natural field of vision

8. Experiment One • Participants walk on a treadmill wearing a HMD with simulated optical flow and try to match the speed of walking to the speed of flow • Participants looked straight ahead • Treadmill ran at 3 mph ( a fast walk )

9. Experiment One: Results • Subjects chose an optical flow corresponding to 4.6 mph to match the speed of walking • Straight-ahead optical flow in the HMD is perceived to be too slow for the actual movement speed

10. Experiment Two • This experiment is the same setup as experiment one, but instead of the subjects looking straight ahead, they look perpendicularly to the direction of motion • Both looking down at the ground • And looking over to a point on the horizon • This maximizes lamellar flow

11. Experiment Two: Results • When looking both down and over, participants chose the optical flow speed that corresponded with the walking speed • the error of their guesses increased with walking speed, but without general over or underestimation

12. Experiment Three • Does the walking simulation cause the misperception of speed? • Subjects walk at normal speed and in “baby steps” • Although perception differs, subjects perceived the optical flow to be closer to correct when using baby steps, so stride-length cannot be used to account for the error in estimation from experiment one

13. Experiment Four • Is misperception caused by image latency or jitter? • Experiment with perception when the eye point is tracked to head movement (which introduces latency) vs. when it is constant. • Result: speed is similarly misperceived with or without jitter and latency.

14. Discussion • Hypothesis is consistent with the effects of different fields of view in speed estimation • Cycling speed underestimated with FOV < 73 degrees, and overestimated with FOV > 103 (Van Veen, et al. 1998; Osaka 1988) • What does this mean for perception of speed in games with variable FOV? • Other functions such as estimation of time to contact and simulated flight accuracy get better with larger FOV (to a point)

15. More Discussion • Some applications are not conducive to turning to the side (e.g. driving sims) • They suggest “adjusting the gain of optical flow” when facing forward. How would this work while maintaining geometric consistency?

16. Questions?