Hack 21. Objects Move, Lighting Shouldn’t
2.10.2. How It Works
Your brain constructs an internal 3D model of a scene as soon as you look at one, with the influence of shadows on the construction being incredibly strong. You can see this in action in the first movie: your internal model of the scene changes dramatically based solely on the position and motion of a shadow.
I feel bad saying “internal model.” Given that most of the information about a scene is already in the universe, accessible if you move your head, why bother storing it inside your skull too? We probably store internally only what we need to, when ambiguities have been involved. Visual data inside the head isn’t a photograph, but a structured model existing in tandem with extelligence, information that we can treat as intelligence but isn’t kept internally.
T.S.
The second movie shows a couple more of the assumptions (of which there are many) the brain makes in shadow processing. One assumption is that darker coloring means shadow. Another is that light usually comes from overhead (these assumptions are so natural we don’t even notice they’ve been made). Both of these come into play when two-dimensional shapesordinary picturesappear to take on depth with the addition of judicious shading [Hack #20] .
Based on these assumptions, the brain prefers to believe that the light source is keeping still and the moving object is jumping around, rather than that the light source is moving. And this despite all the cues to the contrary: the lighting pattern on the floor and walls, the sides of the box being lit up in tandem with the shifting shadowthese should be more than enough proof. Still, the shadow of the ball is all that the brain takes into account. In its quest to produce a 3D understanding of a scene as fast as possible, the brain doesn’t bother to assimilate information from across the whole visual field. It simplifies things markedly by just assuming the light source stays still.
It’s the speed of shadow processing you have to thank for this illusion. Conscious knowledge is slower to arise than the hackish-but-speedy early perception and remains influenced by it, despite your best efforts to see it any other way.
2.10.3. End Note
Zigzagging ball animation thanks to D. Kersten (University of Minnesota, U.S.) and I. Bülthoff (Max-Planck-Institut für biologische Kybernetik, Germany)
2.10.4. See Also
The Kersten Lab (http://gandalf.psych.umn.edu/~kersten/kersten-lab) researches vision, action, and the computational principles behind how we turn vision into an understanding of the world. As well as publications on the subject, their site houses demos exploring what information we can extract from what we see and the assumptions made. One demo of theirs, Illusory Motion from Shadows (http://gandalf.psych.umn.edu/~kersten/kersten-lab/images/kersten-shadow-cine.mov), demonstrates how the assumption that light sources are stationary can be exploited to provide another powerful illusion of motion.
Kersten, D., Knill, D., Mamassian, P., & Buelthoff, I. (1996). Illusory motion from shadows. Nature, 379(6560), 31.
Taken From : Mind Hacks
