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 offers the best available insight into how video refereeing works today. Twelve dedicated cameras are mounted under the stadium roof. They track the ball and up to 29 data points on each player at 50 times per second, calculating the player's exact position on the pitch.

These 29 points cover all limbs and extreme body parts relevant to offside calls. This addresses a problem FIFA itself once described. Studies showed that human operators tend to select different body parts when drawing the offside line, leading to different results for the same situation.

The ball as a sensor

The second element is the ball. Al Rihla, the official World Cup ball, contained an IMU (inertial measurement unit) inside. It transmitted data to the video operations room 500 times per second. This stream allows for precise determination of the moment of the kick, historically one of the most difficult tasks in offside judging.

The system combines limb data with ball data. It then applies artificial intelligence to send an automatic alert to the video referees if a player receives the ball while offside at the moment of the pass. The video referees do not accept this uncritically. They verify the automatically selected kick moment and the automatically drawn line. Only then do they inform the on-field referee. Hence the term "semi-automated" in the name.

The third layer: animation

After the decision is approved, the exact same data points used for the ruling are converted into a 3D animation. It appears on stadium screens and is sent to FIFA's broadcast partners. The effect is twofold: fans see the reasoning, not just the verdict, and the animation always shows the best perspective for that specific situation.

The system was tested at the 2021 Arab Cup and the 2021 Club World Cup, among other events. MIT Sports Lab analyzed the data collected in online and offline tests. TRACK from Victoria University validated the limb-tracking technology, while a research team at ETH Zurich provided insights on such multi-camera systems.

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

FIFA explicitly names the team behind the system. Pierluigi Collina, chairman of the FIFA Referees Committee, and Johannes Holzmüller, FIFA's director of football technology and innovation, announced the technology jointly. Holzmüller describes the setup as generating AI-based offside alerts in real time. He refers to the video referees as the quality control for these results. This is why FIFA still calls the system semi-automated rather than automated.

One often-overlooked fact: FIFA is the sole owner of the tracking and ball data. This is not a technical detail but part of the business model of the entire operation.