The First Measurement: Fixing the Instant the Ball Is Played
Everything downstream depends on choosing the right timestamp, because the positions being compared change from one fraction of a second to the next. An automated system therefore has to detect the moment the ball is played before it can say anything about who was ahead of whom. In practice that means tracking the ball continuously and identifying the point at which its trajectory is altered by contact with a player.
Clean strikes are the easy case. The difficulty sits in the messy ones: a pass rolled along the inside of the boot across several frames, a scuffed touch that redirects the ball gradually, a cross that clips a defender on the way through. Each of those smears the contact across a window rather than concentrating it in an instant, and the system still has to nominate one point inside that window. A small shift in where the timestamp lands can flip a marginal call, which is why contact detection matters at least as much as the body-position measurement that attracts all the attention.
