Offside looks like a simple rule and behaves like a measurement problem. The law itself is short: an attacker in the opponent's half is offside if any part of the body with which a goal can legally be scored is nearer to the opponent's goal line than both the ball and the second-last defender, at the moment a teammate plays the ball. Everything difficult about the decision hides inside that final clause. The rule does not describe a situation that persists. It describes a state that exists for a single instant, and the instant is fixed by an event happening somewhere else on the pitch.
That is why offside resisted technology long after other calls yielded to it. A ball fully crossing a goal line is a continuous physical event a sensor can watch. A foul is a judgment about force and intent. Offside is neither. It is a spatial comparison between moving bodies, evaluated at a timestamp that has to be identified independently of the comparison itself. Semi-automated offside exists because both halves of that problem turned out to be tractable by machine, and because neither was ever going to be resolved reliably from the touchline.
