Illusions and visual special effects – explanations and tutorials

Optical Illusions

Position cues from a moving shadow

March 23rd, 2009 by david




Does the ball sometimes seem to be bouncing, and moving nearer and further away?  Look again just at the track of the ball and you’ll see that all it ever does is to move diagonally from one corner of the board to the other. The spatial effects, and even the way the ball seems to accelerate at points, are all down to the moving shadow. When the shadow sticks to the ball, the ball seems to just move across the surface and into the distance. That’s remarkable, because the ball should appear smaller with distance, but in fact the image of the ball here doesn’t change. The shadow cue is so strong it over-rides the problem. As the shadow drops to the foot of the image, the ball appears higher in the space, but nearer to us. Once again, the effects appear even though the ball does change at all in size, as it should according to the rules of perspective – though some viewers might see an illusion of size-change, compensating for the anomalous lack of real size change.

I’ve tried to base my animation demo pretty closely on one described by Daniel Kersten and colleagues in 1997, in their celebrated original publication of this effect

3 Responses

  1. Unscheduled Post: Visualising 3D « Maxwell’s Demon Says:

    [...] in optical illusions.  The book Mind Hacks has some wonderful examples.  For example how we trust shadows (and assume lights are [...]

  2. Recommended readings (4/27/09) « Division by Zero Says:

    [...] Position cues from a moving shadow ~ who doesn’t love a good optical illusion? [...]

  3. Should your own mind be trusted…As seen through this illusion? Probably not. « Anguished Repose Says:

    [...] is paired with its shadow. So when a fake shadow is introduced into an animation, our brain is primed to link it to the moving object above since in the real world this type of motion wouldn’t arise from independent [...]

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