21 January 2014

Temporal Resolution Addendum

An interesting spinoff of last Friday’s blog post topic, Visual Perception, is the Wagon Wheel Effect or Stroboscopic Effect: A rotating wheel or propeller appears to be stationary, rotate slower than its actual rotation or rotate backwards

Here are four examples, the first two from YouTube, the last two from a discussion of Temporal Aliasing with Cinema.

Movie clip in which a rotating wagon wheel appears to be
stationary: www.youtube.com/watch?v=6XwgbHjRo30.
Video of a hovering helicopter whose propeller
blades appear to be stationary:
www.youtube.com/watch?v=rVSh-au_9aM.
Two videos of a helicopter landing. Propeller blade
rotation appears (1) slower than the true blade rotation,
vimeo.com/66840907 and (2) reversed, vimeo.com/66840906.

Steering wheel with four spokes.
(photo from socalsac.com)
How does it happen? Suppose you make a video of a rotating wheel that has four identical spokes, 90 degrees apart (see photo of steering wheel). If the wheel rotates exactly 1/4, 1/2 or 3/4 of the way around between frames, it would appear in the video that the wheel hasn’t rotated at all. Because the spokes are indistinguishable, it would appear that they haven’t changed position.

Now, if the wheel rotates a bit more than 1/4, 1/2 or 3/4 of the way around between image frames, it would appear in the video that the wheel is rotating but slower than its actual speed. If the wheel rotates a bit less than 1/4, 1/2 or 3/4 of the way around between frames, it would appear in the video that the wheel is rotating backwards. 

Instead of making a video, you could see the same effects with a flashing strobe light that lights up the rotating wheel at the corresponding rotational intervals.

The Wagon Wheel Effect and other forms of temporal aliasing can be avoided by using a frame rate or strobe light rate that’s at least double the speed of the rotating or fastest moving object in the scene.

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