Line Umpire FAQ

I have problems that are not mentioned in this FAQ, can I contact anyone and get help?

Yes, please send an email to
and we will try to help.

Why are not all bounces detected?

It can be because the trajectory detection region is too small. Try making it larger. It can also be because the trajectory detection algorithm is not suitable for your environment. Try changing the confidence and points parameters, in the settings page, as well as the other Trajectory detection settings such as object radius found in the Settings app.

So Line Umpire can be used for tennis. Can it be used for other sports too?

Yes, probably, but it has not been tested. But likely it could be used in other sports where the location of the bounce is central such as volleyball.

I can't hear the out call or signal. What can I do to make it louder?

Yes, unfortunately the sound from the phone disappears a bit in a big room such as a tennis court. A bluetooth speaker or other external speaker could be connected to increase the volume. Also if you have an Apple Watch, make sure to run the application on your watch and start a workout session. Then the watch will vibrate on call, and show a photo of the bounce.

Can Line Umpire be used both indoors and outdoors?

Probably and hopefully. There is a risk that there will be more interfering factors and moving objects such as wind and leaves in trees and so on. However, there are lots of settings to tweak the detection to the environment, so hopefully it will work just as well outdoors. As of now, Line Umpire has only been tested indoors, since it's been developed in Sweden during the fall and winter. Outdoor testing will commence as soon as possible.

Why are things other than tennis balls, such as feet, racquet or people in background, sometimes detected as bounces?

Line Umpire identifies all motion in the view, and then checks if that motion resembles a trajectory. A backswing, or a step might be taken for a trajectory. It's also important that the iPhone is securely mounted and does not move, as that will prevent correct detection. To filter out background motion such as spectators moving or players or balls on adjacent courts, adjust the size and positioning of the trajectory detection region. One way to make these observations not be treated as a bounce is to set limits on the object radius, trajectory distance and trajectory duration.

Why are shadows of players and balls interpreted as bounces and trajectories?

This is a known problem. The app has been tested indoors, where shadows are not as sharp, and no problem with shadows have been observed. A fix for shadows is in development.

Why is there no image of the bounce?

Sometimes the algorithm does not find a frame to associate with the bounce. This can be because the frame where the bounce happened is no longer in memory. You can try to increase the number of frames being stored in the settings.

Can Line Umpire be used on the service line?

Yes it can. But Line Umpire does not yet know the difference between an incoming serve and an incoming normal stroke. And since only the serve should be called out if it's longer than the service line, it's better to disable the flash and the audio, and instead use Line Umpire as a 'Hawk-Eye challenge', and go to the phone and inspect the bounce if either player is uncertain.

Can Line Umpire be used on the center service line?

There is no functionality built in to prevent monitoring of the center service line. But for the security of the phone, it's not recommended to use it for the center service line. It cannot be put on the net side of service line as it would be intefering with the game, and the phone would also be at big risk of being hit. Putting it in the center behind the court could work, but it would also come with a risk that the phone or tripod gets hit and out of position.

Why does Line Umpire only monitor one line?

It's for accurary reasons. Same as a human line umpire, the accuracy gets higher when the phone looks at just one line. This also means that balls that are wide will not be called out by Line Umpire if it's setup to monitor the baseline, just as they would be not be called out by a human baseline umpire.

What is the best way to mount the iPhone when running Line Umpire?

That depends on the size of the area where you want to have bounces detected. When tested the iPhone has been mounted parallell to the baseline about 4-5 meters away and at a height of around 1 meter, and then slightly tilted downwards and forwards into the court. This setup has detected close to all bounces between the service line and 1.5 meters behind the baseline. If used on the sideline, best result has been achieved when mounting the phone on the net side of the sideline, although this of course creates a risk of hitting the phone with a ball. If used as a service line monitor, it could maybe be mounted closer to the service line, and with a smaller trajectory detection region together with a high framerate, in order to get the best images of the bounce for fast serves. But the best result is likely achieved if you experiment a bit with the positioning and settings.

What version of iOS do I need?

iOS 14.5 is the minimum version. However, the system algorithms for detecting trajectories and persons are constantly being refined, so the latest iOS version is recommended.

What iPhone do I need to run Line Umpire?

Line Umpire has been tested on an iPhone 7, and can perform bounce detection at 60 fps. On an iPhone Xs it has been tested to perform bounce detection at 120 fps. And on an iPhone 13 mini, it could detect bounces at 240 fps (which is the maximum framerate) if the region is small enough. So if you want to be able to run bounce detection at 240 fps, an iPhone of the newest model is recommended. If you are OK with running bounce detection at 60 fps, you can use older versions of the iPhone.

Why are there so many settings?

Line Umpire has been thoroughly tested in a few environments and with a few hardware devices. But since all environments are different in terms of ball visibilty, and since the processing capacity of different version of the iPhone differs, a lot of settings are exposed in order for you to optimize the app for your environment and device. Currently a lot of the settings sit in the settings app. They will later be migrated to in-app settings instead.

How do I clear all the bounces?

You can clear the bounces with a swipe to the right across the screen. You can also adjust the settings so that no bounces are drawn, or only the most recent.

What framerate should I use?

That depends on the speed of the incoming balls. For fast balls like serves, it's good to use a smaller region with a higher framerate, whereas for balls which are visible longer, such as balls on the sideline, a bigger region with a slower framerate can be used.

Does Line Umpire drain my battery?

When looking for bounces, Line Umpire does not enter sleep mode, and processes up to 240 video frames per second. So it is more power consuming than the average app. For an hour of constant use on an iPhone 13 mini about 25% of the battery was consumed. More tests will be performed on this.

How accurate is Line Umpire?

Line Umpire works by identifying the lowest point on a trajectory and comparing that point with the line that has been setup. That process has been tested to be quite exact. And since an image is extracted from the frame Line Umpire uses for making the call, you can compare yourself and evaluate its precision. One assumption made by Line Umpire is that it does not take into account the power of the shot, which influences the size of the ball impact on the ground. Instead all balls get the same ground impact size. This will mean some error margin.

Why is the start button disabled?

The start button is disabled until a line has been detected by the automatic line detection, or until a manual line has been setup. So if the automatic line detection does not find your line, you can change mode and setup a line manually by moving the line overlay, rotating it, and changing its size.

Why is my line not identified?

Line Umpire looks for straight lines stretching across the dashed region. If no long enough lines are detected, you can try to change the location of the region and its size. You can also try to adjust the placement of the iPhone.

I think the app is great but why is this nice feature missing?

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