Offside decisions drawn on screen look like geometry, but the hard part is time rather than space. The line can only be placed at a moment the cameras actually recorded.
The decision hinges on a single instant
Offside is judged at the moment the ball is played, not when it arrives. Everything downstream depends on identifying that instant correctly.
A camera does not record time continuously. It samples the pitch at fixed intervals, and each sample becomes a frame with a timestamp attached.
If the ball leaves the boot between two samples, the true moment of contact sits in a gap that no image covers.
Frame rate sets the size of that gap
A system running at a low frame rate leaves a wider interval between images, and the attacker keeps moving through it.
At sprinting speed a player covers a meaningful distance in the time between frames, which is why the same passage can look marginally onside or offside depending on which image is chosen.
Raising the frame rate narrows the gap but never closes it, because sampling is discrete by nature. It reduces uncertainty rather than removing it.
Choosing the kick frame is a judgement
Operators or automated systems must select the frame that best represents contact with the ball. That selection is itself a decision with an error attached.
Ball tracking helps by identifying the point at which the ball's motion changes direction or speed, which brackets the moment of contact more tightly than the eye can.
Even so, the chosen frame is an approximation, and the offside line inherits every bit of that approximation.
Limb position adds a second source of error
The line is drawn from a specific body part, so the system must know where a shoulder or a knee sits in three dimensions.
Skeletal tracking estimates joint positions from multiple camera angles, and occlusion by other players degrades those estimates in crowded penalty areas.
Two small errors, one in timing and one in limb position, combine into a margin that is usually invisible on the broadcast graphic.
The graphic hides the uncertainty it contains
A rendered line looks absolute because it is drawn as a crisp plane across the pitch. Nothing in the image communicates the tolerance around it.
Some competitions have responded by declining to draw lines for margins smaller than the system can resolve, treating those cases as effectively level.
That approach accepts the limits of the measurement rather than presenting a precision the cameras cannot deliver.