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

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)

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

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

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

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

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

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 )

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

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

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

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

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.

Discussion ● Hypothesis is consistent with the effects of different fields of view in speed estimation – Cycling speed underestimated 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)

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?

Questions?