A Perspective on 3-D Visual Illusions
What the Leaning Tower and related illusions reveal about how your brain reconstructs 3-D images.
A Perspective on 3-D Visual Illusions
- 1 In the Leaning Tower Illusion, discovered by Frederick Kingdom, Ali Yoonessi, and Elena Gheorghiu of McGill University, two identical side-by-side images of the same tilted and receding object appear to be leaning at two different angles. This incredible effect was first noticed just last year in images of the famed Leaning Tower of Pisa, but it also works with paired images of other tilted objects.
- 2 The Leaning Tower Illusion elucidates the way in which the visual system uses perspective to help reconstruct 3-D objects. We say "reconstruct" because the visual system has no direct access to three-dimensional information about the world. Our perception of depth results from neural calculations based on several rules. Such rules include: perspective (parallel lines appear to converge in the distance), stereopsis (our left and right eyes receive horizontally displaced images of the same object, resulting in the perception of depth), occlusion (objects near us occlude objects farther away), shading, chiaroscuro (the contrast of an object as a function of the position of the light source), and sfumato (the feeling of depth that one gets from the interplay of in- and out-of-focus elements in an image, as well as from the level of transparency of the atmosphere itself). The Leaning Tower Illusion shows that the brain also uses the convergence angle of two reclining objects as they recede into the distance to calculate the relative angle between them. Thus, two truly parallel towers, such as the Petronas Twin Towers in Kuala Lumpur, appear to converge in the distance as they recede. Two identical pictures of the Leaning Pisa Tower do not result in convergence as they recede, however. So your brain perceives the identical images as non-parallel and diverging. Photo courtesy Thomas Haltner
- 3 Further analysis of similar images reveals subtleties in the way our visual system processes the perception of depth and perspective. For instance, the Leaning Tower Illusion also works with paired images of train tracks, violating the rules of perspective. It's hard to believe, but these are actually identical images of parallel train tracks. Although the angles are the same in both images, the brain perceives them as being quite different. Courtesy of B. Micklethwait
- 4 However, the illusion does not occur when viewing two leaning Japanese manga girls, even though the two cartoon images are tilted. The reason is that the cartoon girls do not appear to recede in depth, so our brain does not expect that they would converge into the distance. This phenomenon demonstrates that the brain only applies its depth perception tool-kit in specific situations. Courtesy of A. Kitaoka
- 5 The leaning tower illusion is such a fundamental feature of our visual system that it works even if one draws a 3-D solid object as it recedes into the distance. The parallel lines give the illusion of diverging in the distance. That is, the box appears wider at the back than it does at the front, when it fact the back and front are precisely the same width on the retina. Courtesy of Frederick Kingdom
