I guess if you wish to have a window area detector you could have two video
ram buffers, one storing a digitised image to be used as a reference, and
the other being actively filled with digitised video from a camera. A PIC
could then be reading out the digitised values from both buffers and
comparing them. If two port ram is used then the video store operation will
not affect the PIC readout operation.

The PIC could then keep track of which parts of the image are inside the
desired detection window, and look only at those ram locations.

It may be possible to dispense with the active video buffer and have a
digital comparator comparing the incoming digitised video with the
appropriate ram location in the reference buffer, and signal the PIC if they
are different by more than say 2 bits to allow for noise. The PIC could then
select the window by timing from the sync pulses, and if the difference
signal occurred inside the window area then raise suitable alarm signals or
whatever.

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