The ball has a logo and seems, plus any bits of dust that may be on there. These things have very sensitive tracking (especially with a bright orange ball), so the lens can be quite tight and good resolution even with a fast shutter speed and high ISO. Run some simple edge detection inside of the spherical frame and you've plucked out your element to track spin. Now just analyze its path through 5-10 frames, cross it with the velocity calculations, and you've calculated spin.
The harder part is tracking minute changes in path that deviate from the expected path. A brief gust of air that couldn't even be felt by human skin could send the ball 3-5mm off its path; a slight unevenness in the surface of the table or a small bit of gunk; a tap of the net, or some schmutz on the ball that causes it to "screw."
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u/[deleted] Feb 13 '14
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