Abstract-Multiple views of a scene can provide important information about the structure and dynamic behavior of three-dimensional objects. Many of the methods that recover this information require the determination of optical flow-the velocity, on the image, of visible points on object surfaces. An important class of techniques for estimating optical flow depend on the relationship between the gradients of image brightness. While gradient-based methods have been widely studied, little attention has been paid to accuracy and reliability of the approach.Gradient-based methods are sensitive to conditions commonly encountered in real imagery. Highly textured surfaces, large areas of constant brightness, motion boundaries, and depth discontinuities can all be troublesome for gradient-based methods. Fortunately, these problematic areas are usually localized can be identified in the image. In this paper we examine the sources of errors for gradient-based techniques that locally solve for optical flow. These methods assume that optical flow is constant in a small neighborhood. The consequence of violating in this assumption is examined. The causes of measurement errors and the determinants of the conditioning of the solution system are also considered. By understanding how errors arise, we are able to define the inherent limitations of the technique, obtain estimates of the accuracy of computed values, enhance the performance of the technique, and demonstrate the informative value of some types of error.

We conducted two experiments that compared distance perception in real and virtual environments in six visual presentation methods using either timed imagined walking or direct blindfolded walking, while controlling for several other factors that could potentially impact distance perception. Our presentation conditions included unencumbered real world, real world seen through an HMD, virtual world seen through an HMD, augmented reality seen through an HMD, virtual world seen on multiple, large immersive screens, and photo-based presentation of the real world seen on multiple, large immersive screens. We found that there was a similar degree of underestimation of distance in the HMD and large-screen presentations of virtual environments. We also found that while wearing the HMD can cause some degree of distance underestimation, this effect depends on the measurement protocol used. Finally, we found that photo-based presentation did not help to improve distance perception in a large-screen immersive display system. The discussion focuses on points of similarity and difference with previous work on distance estimation in real and virtual environments.

Two experiments were conducted to compare distance perception in real and virtual environments. In Experiment 1, adults estimated how long it would take to walk to targets in real and virtual environments by starting and stopping a stopwatch while looking at a target person standing between 20 and 120 ft away. The real environment was a large grassy lawn in front of a university building. We replicated this scene in our virtual environment using a nonstereoscopic, large screen immersive display system. We found that people underestimated time to walk in both environments for distances of 40-60 ft and beyond. However, time-to-walk estimates were virtually identical across the two environments. In Experiment 2, 10-and 12-year-old children and adults estimated time to walk in real and virtual environments both with and without vision. Adults again underestimated time to walk in both environments for distances of 60 ft and beyond. Again, their estimates were virtually identical in the real and virtual environment both with and without vision. Children's time-to-walk estimates were also very similar across the two environments under both viewing conditions. We conclude that distance perception may be better in virtual environments involving large screen immersive displays than those involving head mounted displays (HMDs).

The Lombard effect is the tendency to increase one's vocal intensity in noise. The present study reports three experiments that test the robustness of the Lombard effect when speakers are given instructions and training with visual feedback to help suppress it. The Lombard effect was found to be extremely stable and robust. Instructions alone had little influence on the response to the noise among untrained speakers. When visual feedback correlated with vocal intensity was presented, however, subjects could inhibit the Lombard response. Furthermore, the inhibition remained after the visual feedback was removed. The data are interpreted as indicating that the Lombard response is largely automatic and not ordinarily under volitional control. When subjects do learn to suppress the effect, they seem to do so by changing overall vocal level rather than their specific response to the noise.

Two experiments examined how 10- and 12-year-old children and adults intercept moving gaps while bicycling in an immersive virtual environment. Participants rode an actual bicycle along a virtual roadway. At 12 test intersections, participants attempted to pass through a gap between 2 moving, car-sized blocks without stopping. The blocks were timed such that it was sometimes necessary for participants to adjust their speed in order to pass through the gap. We manipulated available visual information by presenting the target blocks in isolation in Experiment 1 and in streams of blocks in Experiment 2. In both experiments, adults had more time to spare than did children. Both groups had more time to spare when they were required to slow down than when they were required to speed up. Participants’ behavior revealed a multistage interception strategy that cannot be explained by the use of a monotonic control law such as the constant bearing angle strategy. The General Discussion section focuses on possible sources of changes in perception-action coupling over development and on task-specific constraints that could underlie the observed interception strategy.

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This investigation examined short-term changes in child and adult cyclists' gap decisions and movement timing in response to general and specific road-crossing experiences. Ten-and 12-yearolds and adults rode a bicycle through a virtual environment with 12 intersections. Participants faced continuous cross traffic and waited for gaps they judged were adequate for crossing. In the control condition, participants encountered randomly ordered gaps ranging from 1.5 to 5 s at all intersections. In the high-density condition, participants encountered high-density intersections sandwiched between sets of control intersections. These high-density intersections were designed to push participants toward taking tighter gaps. Participants in both conditions were more likely to accept 3.5, 4, 4.5, and 5 s gaps during the last than the first set of intersections, whereas participants in the high-density condition were also more likely to accept very tight 3 s gaps at the last than the first set of intersections. Moreover, individuals in the high-density condition who waited less and took shorter gaps during the middle intersections were also more likely to take very tight 3 s gaps during the last intersections. Ten-year-olds in both conditions had more time to spare when they cleared the path of the oncoming car at the last intersections, whereas 12-yearolds and adults showed no change in time to spare across intersections. Discussion focuses on linking short-term change in perceptual-motor functioning to longer-term perceptual-motor development.Keywords perceptual-motor development; perception-action coupling; road crossing; practice A fundamental problem confronting the developing perceptual-motor system is learning how to bring decisions and actions tightly in line with perceptual information. This ability to fine tune judgments and actions is important both for learning new perceptual-motor skills and for improving existing ones. Becoming a skilled pedestrian, for example, involves improved use of visual information to guide gap decisions and to time interceptive movements. Clearly, experience plays a critical role in producing these kinds of changes in perception-action tuning. Probably the most important aspect of this experience is repeated practice with performing perceptual-motor skills. But how does practice with performing a

We conducted three experiments to compare distance perception in real and virtual environments. In Experiment 1, adults estimated how long it would take to walk to targets in real and virtual environments by starting and stopping a stopwatch while looking at a target person standing between 20 and 120 ft away. The real environment was a large grassy lawn in front of a university building. We replicated this scene in our virtual environment using a nonstereoscopic, large-screen immersive display system. We found that people underestimated time to walk in both environments for distances of 40 to 60 ft and beyond. However, time-to-walk estimates were virtually identical across the two environments, particularly when people made real environment estimates first. In Experiment 2, 10-and 12-year-old children and adults estimated time to walk in real and virtual environments both with and without vision. Adults underestimated time to walk in both environments for distances of 60 to 80 ft and beyond. Again, their estimates were virtually identical in the real and virtual environment both with and without vision. Twelve-yearolds' time-to-walk estimates were also very similar across the two environments under both viewing conditions, but 10-year-olds exhibited greater underestimation in the virtual than in the real environment. A third experiment showed that adults' time-towalk estimates were virtually identical to walking without vision. We conclude that distance perception may be better in virtual environments involving large-screen immersive displays than in those involving head-mounted displays (HMDS). • 217 • J. M. Plumert et al.