It's a scene played out daily on tennis courts the world over.
Although pros have had electronic line calling during matches since 2004, club players and juniors can now buy their own device for roughly the same price as a new tennis racket.
The $199 In/Out promises to put a stop to those endless discussions about line calls once and for all.
The device is the brainchild of French inventor Grégoire Gentil, who spent two years developing it in his living room in Palo Alto, California.
"Detecting the lines of a tennis court is like detecting the lines of the road in the Tesla," Gentil said in an email exchange with CNN.
His device, which is the size of a small camera and will be available this summer, can be set up in less than a minute on top of the net post. It provides real-time line calls.
"I'm an inventor and I understand electronics," said Gentil, an avid tennis player who learned to play on red clay during his youth in Paris.
Although the ball leaves a mark on clay, arguments over which mark is the right one first gave him the idea for the In/Out device.
"I'm aware of the progress of computer vision for the self-driving car," he said. "I knew that it was possible to apply the 'same' algorithms to sports."
Here's how it works:
With a precision of two to three centimeters on average, In/Out may not be as accurate as Hawk-Eye and PlaySight but it doesn't come with the same hefty price tag.
The Hawk-Eye ball-tracking and challenge system, brought in by the grand slam tournaments in 2006 after a number of highly controversial line calls against current women's world No. 1 Serena Williams during the 2004 US Open, costs around $60,000 per court.
PlaySight's SmartCourt system costs up to $12,500 on an outdoor court, with an additional $500 a month for maintenance and cloud storage.
It offers multi-angle, instant high-definition video review and analysis and is used by clubs and elite performance centers across the world as well as top players such as former French Open finalist Simona Halep.
"I'm a tennis player and I'm aware of Hawk-Eye and PlaySight," said Gentil, who holds a Master's degree in mathematics and physics from Ecole Polytechnique in France and a Master's degree of science in engineering management from Stanford University in California.
Other sports to follow?
"I have always dreamed of the same but for any player on any court at an affordable price," said Gentil, who partly funded the development of the In/Out device from a company sale to networking technology giant Cisco Systems in 2004.
Although Gentil is focusing on the market for recreational and junior tennis players, he said he's received "dozens of emails" from volleyball players.
"I have the vision to do a bunch of other sports," said Gentil, whose main goal is to produce a versatile "smart sport camera" that can be used in a variety of games.