Effects of Position and Contour Blurring of Cast Shadow on Depth Perception

Shadows are known as a qualitative depth cue. Moreover, dynamic changes of shadow position were recently reported to lead to the perception of a looming object, suggesting that shadows can be a quantitative depth cue as well. If this is the case, the position and/or contour blurring of shadows would contain quantitative depth information; however, the visual system needs to assume a direction and/or size of a light source to use this information. To examine this idea, we measured perceived depth between a target object and a background for different positions and different contour blurrings of the shadow of the object on the background. The displays were 2-D drawings (18 deg × 18 deg) produced by computer software on a CRT screen and seen through a 3 cm × 3 cm aperture in a sheet of black cardboard. The position of the shadow to the lower right of the object was varied in terms of angular distance between the two. The contour of the shadow was blurred by Gaussian filters of different diameters. We found that perceived depth increased with angular distance, but was independent of contour blurring. This result indicates that the visual system makes an assumption on the direction of the light source, but not the size of it, to derive the amount of depth from the object's shadow.