# Misperceived axis of rotation for objects with specular reflections

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Katja Dörschner visited JLU last week and talked about her work investigating structure from motion with specular reflections and textures (see more info in her recent paper: Doerschner, Fleming, Yilmaz, Schrater, Hartung, & Kersten, 2011). She showed an interesting situation where the axis of rotation of a 3D teapot was misperceived due to the motion of the specular reflections on the surface of the teapot (see: Yilmaz, Kucukoglu, Fleming, & Doerschner, 2011), an effect first demonstrated by Hartung and Kersten (2002).

Using the OpenGL/Psychtoolbox framework I have previously described, I replicated this interesting effect. When you play the following movie, a 3D sphere (with sinusoidal perturbations) is rotated. Note the axis of perceived rotation when the object has specular reflections (1st half of the movie) and when the environment map is “painted” onto the surface (2nd half of the movie).

The physical motion of the object is the same in both cases — the object rotates around the vertical axis. When the object only has specular reflections, it appears to rotate around an oblique 45° axis, but when textured it rotates around the vertical axis. After the break, I show similar effects when the object is rotated around the horizontal axis, 45° axis, and when the spatial frequency of the perturbation is manipulated.

Here is the same object, now rotated around the horizontal axis. We have the same perceived effect, rotation around the oblique 45° axis when only specular reflections are present.

Here is the same sphere, now rotated around an axis 45° off-vertical. Note that the motion for the specular case appears the same when the object is rotated around the vertical and horizontal axis (as shown above) or around the 45° axis (shown below). However, the motion is clearly different when the object is textured.

Neat, right?

Now, here’s a series of videos illustrating the effect for perturbations with different spatial frequencies. Note the changes in perceived object geometry and axis of rotation for low and high frequency perturbations.

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