Turbine Blade Illusion


The images on the right both initially appear to depict a cylindrical horizontal surface, lit from the front so that the upper and lower sides are in shadow. As each animation plays, it becomes apparent that only one of the surfaces has a cylindrical 'C' shaped profile, and is lit from the front. The other surface has an 'S' shaped profile, and is lit from above rather than the front.
Why do the surfaces appear to have the same 'C' shape in frontal view? The pattern of light and dark across each shape is ambiguous, and could be produced by many different combinations of surface shape and lighting direction. In order to rule out many alternatives and arrive at an interpretation, the visual system makes two assumptions: (i) that objects generally have a relatively simple, convex shape - like a beach pebble or tree trunk - rather than some more complex shape; (ii) that light generally falls from above - like sunlight - rather than the side.
In a recent journal article, Robert Lee and I argued that this kind of perceptual effect contributed to the strange appearance of the turbine blade that was installed in Victoria Square, Hull, UK, as part of its City of Culture events.
Adams, W. J., & Elder, J. H. (2014). Effects of specular highlights on perceived surface convexity. PLoS Computational Biology, 10.
Kersten, D., Mamassian, P., & Yuille, A. (2004). Object perception as Bayesian inference. Annual Review of Psychology, 55, 271–304.
Kleffner, D. A., & Ramachandran, V. S. (1992). On the perception of shape from shading. Attention, Perception, & Psychophysics, 52, 18–36.
Langer, M. S., & Bulthoff, H. H. (2001). A prior for global convexity in local shape-from-shading. Perception, 30, 403–410.
Mamassian, P., & Goutcher, R. (2001). Prior knowledge on the illumination position. Cognition, 81, B1–B9.
Marlow, P. J., & Anderson, B. L. (2016). Motion and texture shape cues modulate perceived material properties. Journal of Vision, 16, 5.
Mather, G., & Lee, R.J. (2017) Turbine Blade Illusion. i-Perception, 8 (3), 1-5.