Semi-Automated Offside: The Hard Part Was Never the Line

Manual offside review had two problems, and only one was the line

When VAR first reviewed offsides, the operator did two things: pick the frame in which the ball was played, then draw lines across the pitch from the attacker and the last defender.

Enormous public attention went to the lines — their thickness, their placement, the armpits and toes they were drawn from. Almost none went to the first step, which was much the larger source of error.

Broadcast cameras record at 50 frames per second. Between one frame and the next, a sprinting forward moves roughly 15 centimetres.

So an operator choosing between two adjacent frames was choosing, in effect, between two positions 15cm apart — before any line was drawn. Most of the offside decisions that looked absurd were not line-drawing failures. They were timing failures, presented with the false precision of a graphic.

See how VAR works for the wider review process this sits inside.

The ball sensor is the fix

Inside the match ball is an inertial measurement unit, at the ball’s geometric centre, on a suspension mount that holds it steady without changing the ball’s weight or how it flies.

It transmits 500 times a second, recording acceleration and rotation in three dimensions.

A kick is a violent, instantaneous spike in acceleration. At 500Hz that spike is unmistakable, and it gives the exact instant of contact — not the nearest frame, the instant.

That is the whole breakthrough. The cameras were always able to say where people were. What they could not say was when to look.

It also settles a second question no camera can answer well: which touch was the pass. In a crowded box, a deflection and a deliberate pass look similar from above and are entirely different in law.

The limb tracking

Sixteen optical tracking cameras under the stadium roof follow up to 29 data points on every player, 50 times per second — enough points to place shoulders, hips, knees and feet independently rather than treating a player as a shape.

That granularity is required because offside is judged on the parts of the body that can legally play the ball. A player’s foot can be onside while a shoulder is off, and the rule cares about the shoulder.

For 2026, FIFA went further and scanned all 1,248 players across the 48 squads before the tournament, each capture taking about a second, so the system works from a model of that individual’s actual body rather than a generic skeleton.

Between the tracking and the ball, a match generates more than 150 million data points.

Why it is still called semi-automated

The system does not make the decision. It detects a likely offside, generates an alert, and sends it to the video assistant referee team, who validate the kick point and the tracked positions before informing the on-field referee.

The word “semi” is carrying real weight there, and it is not timidity.

Automated systems fail in ways that are obvious to a human and invisible to the software: an occluded limb, a mis-assigned player, a deflection read as a pass. A human check catches the failure modes the model does not know it has.

There is also an institutional reason. The referee remains the decision-maker in the Laws of the Game, and a technology that overruled them directly would change the constitution of the sport rather than assist it. Compare electronic line calling in tennis, which did take that step — and in doing so abolished the umpire’s authority over line calls entirely.

Football has kept the human in the chain. Tennis removed them.

What it does not fix

Two things, and both are worth saying plainly.

Offside is still a rule with a subjective clause. The technology establishes position and timing perfectly. It cannot determine whether a player in an offside position was interfering with play or with an opponent, which is a judgement about intent and effect that no sensor addresses.

Precision is not the same as satisfaction. A goal disallowed by two centimetres remains infuriating, and being told the two centimetres were measured to the millisecond does not help. The technology has removed the argument about whether the decision was right without removing the argument about whether the rule should be applied that way at all.

That second point is the live one. Now that the measurement is genuinely accurate, the pressure has moved to the law: proposals for a tolerance, or for daylight between attacker and defender, are arguments that only became possible once nobody could blame the lines.

For the rule itself, see the offside rule.