Compound Eye Motion Detector 2D: The Reichardt Correlator

A flat row of simulated ommatidia samples a moving striped stimulus in real time, and each neighbouring pair feeds a genuine Hassenstein–Reichardt correlator — the delay-and-multiply circuit that lets flies, bees and countless other insects detect motion direction without ever forming a focused image. The simulation integrates the correlator's low-pass delay line every frame, so the bar colours, the readout and the scrolling spacetime waterfall are the actual output of the differential equation, not a canned animation. Push the stripe spatial frequency or the angular velocity past the eye's interommatidial sampling limit and watch the classic aliasing failure mode emerge: the correlator locks onto the wrong beat and reports motion running backwards, the same sampling-theorem limit that makes a wagon wheel appear to spin the wrong way on film.