2 minutes reading time (426 words)

How does Hawk-Eye Electric Line Calling Work?

Hawk-Ey_20170912-185425_1

Now used in over 80 tournaments around the world, Hawk-Eye's ITF approved Electronic Line Calling service takes the doubt out of close line calls by using the most sophisticated millimeter accurate ball tracking cameras to identify whether a ball has bounced in our out. In addition Hawk-Eye's SMART Replay technology can deliver instant video replays to assist officials with close decisions on foot faults or line calls on clay courts. Hawk-Eye has been an integral part of tennis since 2002 and continues to deliver innovative solutions for federations, tournaments, broadcasters, sponsors and academies that truly enhance the game for players and fans.  

How Hawk-Eye Works

1) Using cameras, Hawk-Eye finds the exact 3D position of the ball at a series of time intervals leading in to a bounce.

2) From these 3D positions, a trajectory of the ball is calculated.

3) Hawk-Eye uses this trajectory to project where the ball will first make contact with the ground and then how much the ball will compress and skid once it has contacted the ground. From this, the "bounce mark" is determined.

Why a line call can look deceptive on television

Television replays look deceptive because the cameras are at the wrong angle looking down at the ball. The ball also has a lot of motion blur and the cameras do not work at a sufficiently high frame rate to capture the crucial part of when the ball first touches the ground.

To have an understanding of how to interpret these broadcast images, an understanding of what a ball does when bouncing is required. The following images are not from the particular line call, but help to show what happens. Each frame of video in the next sequence is at 1000 frames per second; the above broadcast footage is at 150 frames per second.

What happens to a ball when bouncing

To have an understanding of how to interpret these broadcast images, an understanding of what a ball does when bouncing is required. The following images are not from the particular line call, but help to show what happens. Each frame of video in the next sequence is at 1000 frames per second; the above broadcast footage is at 150 frames per second. 

Just before the bounce 

Ball just touches the line 

The same time has elapsed between this frame and the first frame in this sequence as between a single frame of the broadcast footage.

The ball is still on the ground, but now at least 10 cm (the width of a line) beyond where it first made contact with the ground.  

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Wednesday, 03 March 2021
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