Note: This is a large Flash file (~16MB), so downloading will take some time before the animation runs smoothly.
Visually ambiguous tristability. The top and bottom "cube" animations are identical, except for a half-rotation (in the plane of each frame) and an inversion of the superficial pattern's luminance. Also, the top "cubes" occlude the bottom "cubes".
Despite the simplicity of the construction, the visual percept is bewildering. There is a strong bias toward perceiving two convex cubes (at left and right) even though it is not physically plausible they would rotate in this way and interpenetrate. And the connecting faces for half the cycle appears to be a twisting plane, although the cubes have flat faces (no curved lines in the texture). This kind of twisted plane illusion is new to me.
In two other percepts one of the cubes is convex and one is a concave corner, and the rotation is coherent, as if the full figure was rigid. Although this may be harder to see, it is very stable and more physically plausible, except for "disappearing walls".
A fourth possible percept, of two concave corners, is impossible (for me) to lock onto.
A relate illusion: Two impossible dice [link]
MATLAB, Space Software----------------------
There are no restrictions on use of this animation. Claiming to be the originator or owner, explicitly or implicitly, is bad karma. A link (if appropriate), a note to email@example.com, and credit are appreciated but not required.