[opendtv] Re: Why Europe should choose 720P for HDTV

  • From: Henry Baker <hbaker1@xxxxxxxxxxxx>
  • To: Craig Birkmaier <craig@xxxxxxxxx>
  • Date: Mon, 13 Dec 2004 10:56:18 -0800

Motion blur is used as a "low-pass filter" to avoid
temporal aliasing.

The Nyquist theorem says that you have to sample at twice
the rate of the highest frequency.  Basically, this says
that you have to sample fast enough to get both the high
and the low amplitude of the highest-frequency sine wave.

When this is translated into sequences of frames, this means
that the highest frequency representable sine wave (every other dot)
can't move as much as 1 pixel per frame, else it could be interpreted
as moving in the opposite direction (e.g., backwards wagon wheels
in western movies).

In order to avoid this problem, you have to consider the
speed of motion of an object in the scene.  If the object
is moving at (say) 5 pixels per frame, then you will have
to filter the image of the object to eliminate all its
frequencies with a period higher than 10 pixels.

This is pretty severe filtering, and is probably considered
unreasonably harsh by the computer graphics folks.

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