Method of operation
All Hawk-Eye systems are based on the principles of triangulation using the visual images and timing data provided by at least four high-speed video cameras located at different locations and angles around the area of play.[2] The system rapidly processes the video feeds by a high-speed video processor and ball tracker. A data store contains a predefined model of the playing area and includes data on the rules of the game.
In each frame sent from each camera, the system identifies the group of pixels which corresponds to the image of the ball. It then calculates for each frame the 3D position of the ball by comparing its position on at least two of the physically separate cameras at the same instant in time. A succession of frames builds up a record of the path along which the ball has travelled. It also "predicts" the future flight path of the ball and where it will interact with any of the playing area features already programmed into the database. The system can also interpret these interactions to decide infringements of the rules of the game.[2]
The system generates a graphic image of the ball path and playing area, which means that information can be provided to judges, television viewers or coaching staff in near real time.
The pure tracking system is combined with a backend database and archiving capabilities so that it is possible to extract and analyse trends and statistics about individual players, games, ball-to-ball comparisons, etc.
Applications in sport
Cricket
The technology was first used by Channel 4 during a Test match between England and Pakistan on Lord's Cricket Ground, on 21 May 2001. It is used primarily by the majority of television networks to track the trajectory of balls in flight. In the winter season of 2008/2009 the ICC trialled a referral system where Hawkeye was used for referring decisions to the third umpire if a team disagreed with an LBW decision. The third umpire was able to look at what the ball actually did up to the point when it hit the batsman, but could not look at the predicted flight of the ball after it hit the batsman.[3]
Its major use in cricket broadcasting is in analysing leg before wicket decisions, where the likely path of the ball can be projected forward, through the batsman's legs, to see if it would have hit the stumps. Consultation of the third umpire, for conventional slow motion or Hawk-Eye, on leg before wicket decisions, is not currently sanctioned in international cricket and doubts remain about its accuracy in cricket.[4]
Due to its realtime coverage of bowling speed, the systems are also used to show delivery patterns of bowler's behaviour such as line and length, or swing/turn information. At the end of an over, all six deliveries are often shown simultaneously to show a bowler's variations, such as slower deliveries, bouncers and leg-cutters. A complete record of a bowler can also be shown over the course of a match.
Batsmen also benefit from the analysis of Hawk-Eye, as a record can be brought up of the deliveries batsmen scored from. These are often shown as a 2-D silhouetted figure of a batsmen and colour-coded dots of the balls faced by the batsman. Information such as the exact spot where the ball pitches or speed of the ball from the bowler's hand (to gauge batsman reaction time) can also help in post-match analysis.
Tennis
In Serena Williams's quarterfinal loss to Jennifer Capriati at the 2004 US Open, many crucial calls were contested by Williams, and TV replays demonstrated that some were indeed erroneous. Though the calls themselves were not reversed, the chair umpire Mariana Alves was dismissed from the tournament and subsequent US Opens. These errors prompted talks about line calling assistance especially as the Auto-Ref system was being tested by the U.S. Open at that time and was shown to be very accurate.[5]
In late 2005 Hawk-Eye was tested by the International Tennis Federation (ITF) in New York City and was passed for professional use. Hawk-Eye reported that the New York tests involved 80 shots being measured by the ITF's high speed camera, a device similar to MacCAM. During an early test of the system during an exhibition tennis tournament in Australia (seen on local TV), there was an instance when the tennis ball was shown as "Out", but the accompanying word was "In".[citation needed] This was explained to be an error in the way the tennis ball was shown on the graphical display as a circle, rather than as an ellipse.[citation needed] This was immediately corrected.
Hawk-Eye has been used in television coverage of several major tennis tournaments, including Wimbledon, the Stella Artois at Queens, the Australian Open, the Davis Cup and the Tennis Masters Cup. The US Open Tennis Championship announced they would make official use of the technology for the 2006 US Open where each player receives two challenges per set.[6] It is also used as part of a larger tennis simulation implemented by IBM called PointTracker.
The 2006 Hopman Cup in Perth, Western Australia, was the first elite-level tennis tournament where players were allowed to challenge point-ending line calls, which were then reviewed by the referees using Hawk-Eye technology. It used 10 cameras feeding information about ball position to the computers.
In March 2006, at the Nasdaq-100 Open, Hawk-Eye was used officially for the first time at a tennis tour event. Later that year, the US Open became the first grand-slam event to use the system during play, allowing players to challenge line calls.
The 2007 Australian Open was the first grand-slam tournament of 2007 to implement Hawk-Eye in challenges to line calls, where each tennis player on Rod Laver Arena was allowed 2 incorrect challenges per set and one additional challenge should a tiebreaker be played. In the event of an advantage final set, challenges were reset to 2 for each player every 12 games, i.e. 6 all, 12 all. Controversies followed the event as at times Hawk-Eye produced erroneous output. In 2008, tennis players were allowed 3 incorrect challenges per set instead. Any leftover challenges didn't carry over to the next set. Once, in one of Amélie Mauresmo's matches, she challenged a ball that was called in, Hawk-Eye showed the ball was out by less than a millimeter but the verdict was called in. As a result, the point was replayed and Mauresmo didn't lose an incorrect challenge.
The Hawk-Eye technology was used in the 2007 Dubai Tennis Championships with some minor controversies. Defending champion Rafael Nadal accused the system of incorrectly declaring an out ball to be in following his exit. The umpire had called a ball out; when Mikhail Youzhny challenged the decision, Hawk-Eye said it was in by 3mm.[7] Youzhny said afterwards that he himself thought the mark may have been wide but then offered that this kind of technology error could easily have been made by linesmen and umpires. Nadal could only shrug, saying that had this system been on clay, the mark would have clearly shown Hawk-Eye to be wrong.[8] The mark left by the ball on a hard court is a subset of the total area that the ball was in contact with the court (a certain amount of pressure is required to create the mark)[citation needed].
The 2007 Wimbledon Championships also implemented the Hawk-Eye system as an officiating aid on Centre Court and Court 1, and each tennis player was allowed 3 incorrect challenges per set. If the set produced a tiebreaker, each player was given an additional challenge. Additionally, in the event of a final set (third set in women's or mixed matches, fifth set in men's matches), where there is no tiebreak, each player's number of challenges was reset to three if the game score reached 6-6, and again at 12-12. Teymuraz Gabashvili, in his 1st round match against Roger Federer, made the first ever Hawk-Eye challenge on Centre Court. Additionally, during the finals of Federer against Rafael Nadal, Nadal challenged a shot which was called out. Hawk-Eye showed the ball as in, just clipping the line. The reversal agitated Federer enough for him to request (unsuccessfully) that the umpire turn off the Hawk-Eye technology for the remainder of the match.[9]
In the 2009 Australian Open fourth round match between Roger Federer and Tomáš Berdych, Berdych challenged a call that was out. The Hawk-Eye system wasn't available when he challenged, likely due to a particularly pronounced shadow on the court. As a result, the original call stood.[10]
In the 2009 Indian Wells Masters quarterfinals match between Ivan Ljubičić and Andy Murray, Murray challenged a ball that was called out. The Hawk-Eye system showed the ball landed on the center of the line despite instant replay images showing that the ball was clearly out. It was later revealed that the Hawk-Eye system had mistakenly picked up the second bounce, which was on the line, instead of the first bounce of the ball which was clearly out.[11] Immediately after the match, Murray apologized to Ljubicic for the call, and acknowledged that the point was out.
The Hawk-Eye system was developed as a replay system, originally for TV Broadcast coverage. As such, it initially couldn't call ins and outs live, only the Auto-Ref system could produce live in/out calls as it was developed for instant line calling. Both systems can produce replays.
The Hawk-Eye Innovations website states that the system has an average error of 3.6 mm. The standard size of a tennis ball is 65 to 68 mm. This means that there is a 5% error relative to the diameter of the ball. For the sake of comparison, approximately 5% of the diameter is the fluff on the ball.
Unification of rules
Until March 2008, the International Tennis Federation (ITF), Association of Tennis Professionals (ATP), Women's Tennis Association (WTA), Grand Slam Committee, and several individual tournaments had conflicting rules on how Hawk-Eye was to be utilized. A key example of this was the number of challenges a player was permitted per set, which varied among events.[12] Some tournaments allowed players a greater margin for error, with players allowed an unlimited numbers of challenges over the course of a match.[12] In other tournaments players received two or three per set.[12] On March 19, 2008, the aforementioned organizing bodies announced a uniform system of rules: three unsuccessful challenges per set, with an additional challenge if the set reaches a tiebreak. The next scheduled event on the men and women's tour, the 2008 Sony Ericsson Open, was the first event to implement these new, standardized rules.
Snooker
At the World Snooker Championship 2007, the BBC used Hawk-Eye for the first time in its television coverage to show player views, particularly in the incidents of potential snookers.It has also been used to demonstrate intended shots by players when the actual shot has gone awry. It is now used by the BBC at every World Championship, as well as some other major tournaments. The BBC uses the system sporadically, for instance in the 2009 Masters at Wembley the Hawkeye was at most used once or twice per frame. In contrast to tennis, the Hawkeye is never used in snooker to assist referees' decisions.
Association football
The Hawk-Eye has been proposed for use in Association football but has yet to win general approval from the major governing bodies of the sport.
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