12 cameras, 29 body points, 50 Hz: how FIFA measures offside

FIFA published a description of the system after the World Cup in Qatar. It remains the best available look at how video refereeing works today. Twelve dedicated cameras sit under the stadium roof. They track the ball and up to 29 data points on each player, 50 times per second, and work out exactly where that player stands on the pitch.

The 29 points cover every limb and the extreme parts of the body that matter in an offside call. They answer a problem FIFA itself once described. Studies showed that human operators tend to pick different body parts when they draw the offside line. The same situation can then produce different results.

The ball as a sensor

The second element is the ball. Al Rihla, the official World Cup ball, carried an IMU (inertial measurement unit) inside it. It fed data to the video operations room 500 times per second. That stream pins down the moment of the kick, historically one of the hardest tasks in an offside call.

The system combines the limb data with the ball data. It then uses artificial intelligence to send an automatic alert to the video referees when the ball reaches a player who was offside at the moment of the pass. The video referees do not take that alert at face value. They check the automatically selected kick moment and the automatically drawn line, and only then do they inform the referee on the pitch. Hence "semi-automated" in the name.

The third layer: animation

Once the decision is confirmed, the very same data points behind the ruling become a 3D animation. It goes to the stadium screens and to FIFA's broadcast partners. The fan sees the reasoning, not just the verdict. The animation also always shows the best angle for that particular situation.

The system was tested at the 2021 Arab Cup and the 2021 Club World Cup, among other events. MIT Sports Lab analysed the data gathered in online and offline tests. TRACK of Victoria University validated the limb-tracking technology. A research team at ETH Zurich supplied the conclusions about multi-camera systems of this kind.

We know that offside checks sometimes take too long, especially when the situation is very tight, said Pierluigi Collina, chairman of the FIFA Referees Committee.

FIFA names the team behind the system directly. The technology was announced jointly by Pierluigi Collina, chairman of the FIFA Referees Committee, and Johannes Holzmüller, FIFA's director of football technology and innovation. Holzmüller describes the setup as generating AI-based offside alerts in real time. He calls the video referees the quality control on those results. That is why FIFA still calls the system semi-automated rather than automated.

One more fact often gets skipped: FIFA is the sole owner of the tracking data and the ball data. That is not a technical detail but part of the business model behind the whole operation.