Implement a sample & hold with the sample keyed by H and V sync related
windows, then digitize the signal (using PIC ADC). On the next frame
compare this to what you stored previously. The S&H does the 'hashing' in
the form of integration. You could add a peak or through detector before
it do detect dark or light objects better.

The simplest such thing uses a photodiode in a black vacuum suction cup
stuck to a monitor screen and a window comparator to detect changes.

Peter

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