Why can nobody in football draw a straight offside line?

Does this computer-generated offside line look parallel to you? Judging an offside decision on TV just got a whole lot harder...

There is no one more maddening decision in football than an offside goal, but when even the TV companies can’t seem to draw a straight offside line, how are the rest of us meant to know if a goal should stand or not?


Two decisions this past weekend have got fans fuming: one during the Italian Serie A game between AC Milan and Juventus, and the other in the Premier League north London derby between Spurs and Arsenal.

Juventus striker Carlos Tevez scored a goal that Milan fans felt shouldn’t have stood, while Mezut Ozil opened the scoring for the Gunners – even though some replays appeared to show he was offside.

To make matters worse, screenshots from the TV coverage seemed to prove that the computer-generated offside line on the TV replays weren’t drawn straight.

AC Milan’s tweet translates as, “In your opinion, in the still TV image produced by Juventus, are the two lines parallel? Not for us”.

If we don’t believe the linesman, and can’t trust the virtual graphic, how can we ever know if the offside call is correct?

There’s a precedent to this. During last year’s World Cup, Fifa admitted that a TV graphic that apparently showed Brazilian striker Fred offside was in fact wrong. Fifa’s director of television Niclas Ericson said at the time: “It is very fast decisions to put in that [graphic], but mistakes can happen.”

Not particularly reassuring.

But wait! The offside lines might not be wrong after all.

It all comes down to perspective. In the AC Milan footage, the camera was focused on the halfway line, and not parallel to the actual incident. The angle you see the action from changes whether the player looks offside or not.

That angle also affects the TV-produced offside line. The computer generation takes into account the position of the camera when the line is drawn. Because of that, the resulting line sometimes doesn’t look quite right.

The video below (in Italian) from a Sky Italia broadcast shows what’s going on (watch from 2 minutes 15 seconds in):

The same applies for the BT Sport example (warning: slightly technical explanation of how sport computer graphics work coming up).

BT Sport’s graphics are provided by sports graphics tool Piero. The system works by first creating a 3D model of the football pitch based on information from the video feed. Pitch markings, players… it’s all in there.

When the broadcaster wants to draw an offside line, an operator will find the exact frame when the ball is kicked and then click on the last defender. The computer system then draws a line automatically from that player, taking into account the camera perspective and all the other information it has picked up from the 3D model to make sure the line it draws is parallel to the goal line.

The only human input comes with deciding exactly when the ball is kicked, and who the last defender is. In short, it’s not like some producer in a van outside the ground is drawing a line by hand.

Got that? Now when you scream at the telly at least you’ll know how it works.

The blog ‘Offside Explained’ is even better, showing how the position of the assistant referee causes mistakes to be made, and how TV camera angles influence whether something ‘looks’ offside or not. Because the camera isn’t parallel to the action, the offside line can appear skew-whiff.

Here’s just one example.

What do you think? Is Gervinho offside?

The blog post has the answer. “If the camera would be aligned with the ball then we could see that there is no offside,” the site explains. It’s the TV angle that deceives us.

One angle make it look like he’s off…

Another appears to show him clearly on…

It’s all down to the angle the camera (and the assistant referee for that matter) sees the action.

Neat, huh? If you’re bored, you can test yourself with these Fifa training vids.

This is why, in the screen grabs from the weekend, the line drawn by the broadcasters don’t look parallel. Unlike viewers and assistant referees, the computers that generate the offside line are really, really good at geometry, and can work out the proper offside line even when the camera perspective makes it look badly drawn.

That doesn’t mean mistakes still can’t happen, as Fifa in the past have admitted. It’s just that a couple of grainy screen grabs on Twitter don’t show the full picture.


Turns out the camera does lie after all.