Hawk eye or virtual eye, eagle eye, ball tracking, has many names and is the foundation of umpire review systems, but what is it really?
Now you may not be familiar with this particular piece of technology, but cricket and tennis fans know this omnipresent umpire all too well. Hawk eye essentially uses high performance and high speed cameras to capture the motion of the ball and feeds a computer model this information in order to simulate the balls path.
The point of this song and dance is different based on the sport. In tennis it can be used by players to challenge line calls made by the on-court officials. The technology has also found its way into the English Premier League where it is essentially used to review if a ball crossed the goal line or not. Lastly, it is also used in cricket to review controversial leg-before-wicket decisions. Here the rule is the player is out if the ball would have hit the wicket behind them had their leg not been present. This is incredibly hard to judge on the field and thus players from either team have the ability to review the decisions made.
However, this technology is not without controversy. There is a whole slew of cases where the question of ball being in or out has fooled Hawk-eye. The system doesn’t claim to be perfect, nor does it need to be. It just needs to be superior to human referees, and hopefully I can convince you of that fact. Still there are organizations, I am looking at you FIFA, that refuse to use the technology unless it is 100% accurate.
Ball tracking was originally developed as a way to show the television audience the trajectory of the ball more clearly. This meant the technology had to accurately figure out where the ball was at all times and relay that information to humans. These design goals are clearly visible in the design of the technology.
The array of cameras take several hundred pictures every second and look for the ball in each on of these images. A computer then uses the positional data from each camera to figure out how the ball is moving. Each camera takes a 2D image, but they are all pointed at the ball from different places. By tracking the ball through the various frames the computer can work out where the ball is in 3D space from all of these images. Because of the different positioning of the cameras, basic things will change in each image giving the computer clues about the ball’s position. Things like colour, distance, and the shadow the ball is casting are all used in the simulation. Once the computer has a whole bunch of points in 3D, it draws a smooth curve between them using our good old laws of motion from Sir Isaac Newton.
Once we have a good profile of the ball we can talk about the accuracy and precision of the results. The point of contention is the construction of those three dimensional points. To get a perfectly precise point you would need an impractical amount of cameras taking trillions of photos per second. This is because the computer has to make approximations to look at points in between the pictures it has, leading to a limit in precision. So every single reviewed decision has some margin of error attached to it.
Tennis ball impact plus margin of error from Hawk-eye
In a sport like tennis the precision of the hawk-eye instrumentation used amounts to the fluff of the ball (3.6mm spread). This system is clearly going to exceed any human on-lookers’ ability to judge the ball (human resolving power is no where near that accurate [https://en.wikipedia.org/wiki/Visual\_acuity]). In cricket, ball tracking allows the television umpire to look at information that is impossible for the on-field umpire to gain access to. Hawk-eye here is predicting where the ball will end up by using its actual trajectory, rather than the umpire’s ability to divine where the projectile moving at 145km/h was going to end up.
Overturning an on field decision in cricket
The bottom line of Hawk-eye is this: the technology will never be perfect, and fans should be made aware of this. We can use this amazing piece of science to show people the importance of understanding accuracy and precision in modern research. But the reality remains that Hawk-eye is going to get the decision right far far more often than any human referee.
Maybe the only one capable of beating Hawk-eye is Hawkeye.