Angle of regard influences slant perception independent of distance

It is well established that observers overestimate the surface orientation of geographical, virtual, and man-made hills. We investigated whether the <i>v'</i> theory-that observers use the angle of regard-or the relationship between the direction of gaze and the slope of the hill, to make their slope estimates. We tested whether the perceived steepness of hills changes across dramatic differences in eye heights across a wide range of surface orientations, while controlling for distance of the surface from the observer. We found that people use the angle of regard to make their slope estimates across a wide range of surface orientations and eye heights while controlling for distance, standing on the surface, and posture. The dramatic manipulation in eye height caused corresponding changes in slope perception as predicted by the angle of regard. The angle of regard seems to be a perceptual regularity that is constant across changes of position of the observer and surface slope, and also predicts observed changes in eye height and distance of the surface from the viewer.