How are offside lines are drawn by VAR in football?


How are offside lines are drawn by VAR in football?

In: Technology

They know exactly where the camera is, its focal length, and where the pitch is. With these three bits of information you can perfectly overlay a line between the two longer sides of the pitch and slide it up and down, knowing that it represents a line perfectly parallel to the two shorter sides.

I work in visual effects and we often have to do a slightly more complicated version of the same thing called camera tracking – this is where some software goes through a video clip frame by frame, looking for similar clusters of pixels in each one (for example a magazine cover on a table, a pattern on the table, the corner of the table etc). By tracking all these different elements and seeing how they move in relation to each other, it can make a pretty good guess of where the camera is moving – especially if you’re able to tell it the focal length of the camera and provide it with any “known distances” (such as the table width) as these impact perspective and thus where things might appear. Once you have a decent match, you can make a little 3D cube (or any sort of object – in practice what you usually want is some sort of 3D character or background environment) and move it around “on” the table and the perspective of it will match that of the table perfectly, because it’s being shown through a digital equivalent of the same camera, at the same angle, at the same distance as the one actually used to shoot the video clip.

The VAR system works on the same basis, only everything is known before hand – if you (visibly) render those lines on an (invisible) representation of the pitch, using a digital camera identical to the real one, it’ll overlay perfectly. You can then slide it up and down so that it’s touching the foot or shoulder or whatever of the player, and you’ll know that the rest of the line is perfectly flat and parallel too.