The contributions of surface features and contour shapes to object slant perception.

Humans perceive 3D shapes even from 2D images. A slant can be perceived from images of slanted rectangular objects, which include texture gradients and linear perspective contours. How does the visual system integrate and utilize these pictorial depth cues? A new visual illusion that provides some insights into this issue was examined. A box-like object with disk figures drawn on its upper surface was rendered in a linear perspective image. The length of the object's upper surface side line was overestimated, probably due to the foreshortened disks serving as slant cues. This illusory effect occurred even when observers estimated the line length on the image plane, suggesting that the slant perception from the disks was mandatory. Five experiments revealed that multiple depth cues were utilized for the slant perception; the aspect ratio of the disks, texture gradients, trapezium/parallelogram contours, and the side surfaces of the box-like object. However, foreshortened disks outside the object were not utilized as depth cues. These results suggested that various depth cues belonging to the target object are integrated for the slant perception.