Kersten (1997) showed that the perceived motion path of an object in a 3D scene can be manipulated by changing the path of a shadow. Using a scene similar to Kerstena??s "ball-in-a-box" scene, we considered two other variables affecting the perceived path: changes in projected size and changes in projected speed. We expected an object to appear to move higher if the projected size or speed increased while ground contact indicated recession in depth. Each scene began with a stationary ball on a platform at the lower left of a surface extended in depth with two vertical sidepieces. The ball moved toward the upper right of the surface and disappeared. Observers were asked to mark the height at which the center of the ball would have hit the right sidepiece. The balla??s projected path was identical in each display. There were three levels of projected size change: decreasing (consistent with recession in depth), constant (consistent with motion in 2D) and increasing (the reverse of the decreasing function). The three levels of speed change were the same. The judged height at which the ball would intersect the right sidepiece was affected significantly by both the projected size and speed functions. Judged height was greater with increasing speed than with either decreasing or constant speed. Judged height was greater with increasing projected size than with constant projected size and was greater with constant projected size than with decreasing projected size. In addition, the final size and speed were important in determining the judged height at the end of the motion sequence with greater height judged for larger final projected sizes and speeds. These results indicate that size and speed changes inconsistent with the depth change indicated by ground contact results in the perception of an object as rising above the ground surface.
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