Line Umpire

Use your iPhone as a Line Umpire when you play tennis to call a line in real time during play.
Please note: Line Umpire works better on newer devices, where it can make more observations and perform more calculations in less time. Also note that the app monitors one line. So if you want to cover more lines, more devices are needed.
First select the line to monitor, and then as soon as an out ball is detected, the phone will flash and make a sound. Also, If you wear an Apple Watch, your watch will vibrate when the ball is out, and you will see a snapshot image of the last bounce. 
You can connect up to eight devices to each other during a line calling session. That means that when one device detects a ball which is out, all devices will flash and call out. And of course send the bounce to their paired Apple Watch if there is one.
It's pretty awesome already in text format, but should really be experienced in real life!
On the Apple Watch application you can start a HealthKit workout session in order to track heart rate and calories burned. Having an active workout will also enable continuous delivery of line call images to the wrist. When the workout is ended, it is saved using HealthKit and details about it can be found in the Fitness app.
Line Umpire leverages on-device intelligence, and does not use any internet resources. That means it's quite fast and that no data is uploaded anywhere. Cool! It also means it demands quite a lot of your device. Using it for a long time will consume some of the device's battery.
Also, there is no sign up or sign in, just line calling straight away!
For each bounce you can watch a frame by frame replay to verify the line call.
The detection algorithms can be tweaked by lots of settings, so that you can adapt the detection for your specific environment.