Thursday, February 5, 2009

Wheels spinning in opposite direction

Have you ever seen a car with fancy wheels where
the spokes seem to be moving in the opposiste direction
to that of the movement of the car? Or a old western
movie where the wagon wheel spokes seem to be moving in
the opposite direction?

- This is due to alaising. Our eye is not able to sample
fast enough to capture the movement accurately. In case of
the movies the old cameras had a very low sampling frequency.
The nyquist criterion requires sampling frequency to be
at least twice the signal to avoid aliasing.

Wednesday, February 4, 2009


Its a nice ancient art of sending messages through picture.
It relies on providing security through obscurity. Eg:
Changing the color of every 50th pixel in a picture to
represent an alphabet.

Very interesting.