I want to measure the amount of movement in a room. 

INPUT : in the center of the ceiling of the room (that measures
10x10m)there will be half a mirroring ball. This ball will reflect the
room. A mini dv camera (sharp viewcam z 1.3 megapixel)will be
installed under the mirroring ball to film it. the images captured in
this way will be fed to my computer. I am looking for software that is
able to compare image n to image n-5.

OUTPUT : a signal will be generated that is proportional to the amount
of pixels changed. This signal will be fed to a valve that opens and
closes proportionally.

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Clarification of Question by berty-ga on 27 Jan 2005 05:43 PST

The set up as proposed in my question will not deliver the desired
result. As the image of a person closer to the camera is built up of
more pixels that the image of the same person closer to the camera,
the same movement will not cause the same amount of pixels to change.
Therefor the setup must contain multiple cameras. The room will be
divided in 4 zones.
So i am looking for software that can compare the images (at time n)
of 4 different cameras with their past (at time n-5). The sofware must
be able to bundle the result into one signal.