Abstract
When the shadows of a group of linear columns projected on a stepped three-dimensional (3D) plane are diagonally viewed, the retinal image shows a geometric pattern that is similar to the Café wall illusion, which is a popular geometric illusion. In this study, we investigated the relationship between the geometric structure of the scene, such as the 3D shape, viewing angle, and direction of the light source, and the strength of the Café wall illusion that was perceived from the retinal image of those 3 D situations. Consequently, the difference in the depth at which the shadows fall should be approximately 1%–2% of the observation distance to result in a strong Café wall illusion. These results show that the information of the shadow that falls on the object is an effective clue for estimating the shape of the object surface with relatively small depth differences in the case of large viewing distances.
