Visit just about any tennis club on a Saturday, and youre likely to witness otherwise sensible adults losing their minds over line calls. Players suffer complete meltdowns as they hurl insults. Parents morph into brooding teenagers. Friends become enemies. Postmatch beers can undo some of the damage, but the shame and resentment linger for days.

More civilized times may lie ahead. French inventor Grgoire Gentil has designed a $200 GoPro-size device that can be fastened to any net post and detect whether balls are in or out with surprising accuracy. Its called, reasonably enough, the In/Out. I was born in Paris and raised on clay, Gentil says. On clay, the ball leaves a mark, and he recalls many arguments over a blemish on the court. It was the starting point of this, I would say.

Gentil, 44, now lives in Palo Alto and built the In/Out in his living room lab. The device monitors both sides of a tennis court using a pair of cameras similar to those found in smartphones. After attaching the In/Out to the net with a plastic strap, a player pushes a button on its screen, and it scans the court to find the lines using open-source artificial intelligence software. AI also helps the device track the balls flight, pace, and spin. This would not have been possible five years ago, Gentil says.

The In/Outs dual cameras map the lines of a tennis court, and the device beeps to signal missed shots.

Source: In/Out

In a test at Stanford, Gentil and I played for an hour, and the In/Out beeped whenever one of his shots sailed long or wide. (I dont remember missing any.) On close calls, we rushed over to watch a video replay on the In/Out screen. At hours end, Gentil whipped out a tablet and connected to the In/Out app, which showed where all our shots had landed and provided some other stats.

Although equipment like the In/Out has been around for years, Gentils is the only one that costs about as little as a decent racket. Top tournaments, including the Grand Slams, use Hawk-Eye, a Sony Corp.-owned system of superaccurate cameras that customers say costs $60,000 or more to set up on each court. Given the price, its typically reserved for show courts. Sony didnt respond to requests for comment.

PlaySight Interactive Ltd., a startup in Israel, makes a six-camera system thats less accurate than Hawk-Eye but costs a mere $10,000 per court, plus a monthly fee to collect data that can be reviewed online or in an app. PlaySights setup also includes a large screen that lets players see line calls and ball speed without interrupting the game. The company has sold its gear mostly to tennis clubs and universities.

Its screen can show video replays.

Source: In/Out

Chris Edwards heads the product testing work done by retailer Tennis Warehouse and has tried all three tracking systems. The In/Out doesnt bring the same depth of insight as PlaySight, he says. But as far as a portable, cheap device goes, the In/Out has the potential to be the best by far. I havent seen anything else like this.

Over the past decade, Gentil has made a dozen products. He sold a software company to Cisco Systems Inc., designed an augmented-reality motorcycle helmet, and built a hand-size drone that can follow a person around. He spent two years developing the In/Out, tuning the software, even 3D-printing a plastic tennis ball-shaped case for it. Its been a tumultuous process, Gentil says. You get an algorithm working on the tennis court one day and think you will sell hundreds of thousands of units, and the day after, nothing is working.

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Gentil acknowledges his machines limits. The In/Out has a 20-millimeter margin of error, compared with about 3mm for the Hawk-Eye, and can get confused during doubles matches if the extra players block its line of sight. Gentil says he hopes to improve the devices accuracy and recommends that two In/Outs be used for doubles. As for the possibility that Sony or PlaySight might sue him over the concept of his invention, hes filed some patents himself, he says. If Hawk-Eye is coming after me tomorrow morning, they are going against innovation and against the tennis community. I think I might have the tennis community with me.

The bottom line: Gentils $200 line-calling AI isnt as accurate as rival products, but unlike them, its affordable enough for mass adoption.

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This $200 AI Will End Tennis Club Screaming Matches - Bloomberg