Your signature, while technically accurate, could do with an update by the way.

I think they can do this very easily but it will take a few minutes, doing what I did and getting the computer to work out the vanishing points etc, so we could be waiting quite a while until they have got the correct prospective lines sorted out. I think that is the problem!

From what fixed point ?

To do it properly you'd have to accurately place the position of the camera relative to the player in question and then make the adjustment for perspective, easy enough to do given time but virtually impossible to do instantly upon demand during any of the seconds of play available.

The points of reference would change in every ground too so you'd have to have several fixed camera positions in every SL ground that do not change through the season, once the fixed position was known you'd only need to measure the distance from the player to the camera and then let the 3D software work out the perspective before drawing the line.

Or we could just rely on two referees making their own mind up by looking at the TV coverage several times and then listening to what the two clowns in the commentary box say - then go for the opposite decision.

All you would need to do is keep the super imposed lines parrallel to the nearest marked line, ie to the try line or the 20m line and draw them right across the pitch........I think !

Assuming that your camera was postioned on one of the lines yes, but if it was five metres in front of the line as the Sky photo appears to be then the only true line is the one that starts at the camera lens and goes straight up the photo, all the other lines to either side of that are skewed by the position of the lens relative to the players foot and/or the ball.

I'd say its impossible to do with any more accuracy than "best guess" that the referees use, doesn't stop Sky trying to prove a ridiculous theory with a line that would actually be several inches wide if it was real though