Active regulation of altitude as a function of optical texture

Abstract

Two empirical studies are reported that examine active regulation of altitude as a function of the type of ground texture. Three ground textures were examined: lines perpendicular to the direction of motion, lines parallel to the direction of motion, and the combination (i.e., square or checkerboard texture). Although subjects only controlled altitude, disturbances were introduced on three axes: vertical, lateral, and fore-aft. The results show a clear advantage for texture parallel to the direction of motion. However, in considering these results in the context of previous research on altitude control, the argument is made that there is no compelling evidence that suggests either parallel (splay) or perpendicular (density) texture is privileged with regard to altitude control. Rather, the most effective display for altitude control will be the one that best isolates the optical activity associated with changing altitude from the optical activity arising from other sources of disturbance (such as forward locomotion). Such a display will make it easier for the observer to distinguish and respond specifically to the disturbances of altitude.Gibson, Glum, and Rosenblatt (1955) argued that "motion perspective" provided "a basis for the judgments required for the control of locomotion" (p. 385). Motion perspective refers to the flow of texture elements within the optic array of a moving observer. The research reported here focused on one aspect of the visual control of locomotion-regulation of altitude. Two sources of information within the optic array were examined: optical splay angle and global optical density (or optical depression angle). We begin by presenting an analysis of the optical geometry to show the linkage between altitude and optical splay angle and optical depression angle. Next, we review previous research where there is currently debate about the relative efficacy of these two sources of information. We then present two empirical studies that examined these two sources of information in an altituderegulation task. Finally, we attempt to integrate our results with those from previous studies and discuss the theoretical and practical implications. OPTICAL ANALYSISThe term optical splay was introduced by Warren (1982). Warren cites Biggs (1966), who noted that when an observer maintains a constant distance to a line on the ground plane (e.g., the curb of the road), despite the shifting optical positions of the individual points composing the line, the optical position of the line was invariant. For a straight line parallel to the direction of motion, the invariant optical position can be defined in terms of the angle at the vanishing point formed by the line and a second line perpendicular to the horizon along the ground trace of forward motion, as shown in Figure 1. Thus, it can be defined by the equation:where S is the splay angle, Y I is the lateral displacement of the line from the perpendicular, and z is the altitude (eyeheight) of the observer. This equation describes the projection onto the f...

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