In solving the ill-posed problem from recovering 3D form from 2D images
our brain makes a priori assumptions about the world.
Assumption 1 Faces are convex.
In the rotating mask illusion we have a hard time to see a hollow face
mask because our everyday experience tells us that the nose is pointing outwards and not
inwards.
Rotating mask (MPEG1 Video 286
KB)
Assumption 2 Light sources are stationary.
If the cast shadow of an object is moving, we assume that the object is
moving and not the light source. In collaboration with Dan Kersten, Minnesota, USA
A moving shadow induces a motion in depth of a stationary object (green square is not
moving).
Green square (MPEG1 Video
50 KB)
Zigzagging ball?
Zigzagging shadow motion induces apparent zigzagging motion of an object.
We do not see the correct linear trajectory of the blue ball although
changes in shadowing on the ball and on the checker board indicate that the
light source is moving. The yellow line allows us to verify that the path of
the ball is indeed rectilinear.
Blue ball
(MPEG1 Video 333
KB)
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