How Throw scores your darts with a single phone

You stand your phone beside the board, you throw, and the score appears on its own. No cameras bolted around the board, no extra hardware, no server. Here's what happens, in plain terms, between your dart landing and the score showing up.

Step 1: find the board (once)

Before it can score anything, Throw has to know where the board is in the picture and what angle it's seeing it from. Filmed from the side, a round board turns into a squashed oval, and the image needs to be "straightened out".

This is the only step that uses artificial intelligence. A small model, running on the phone, finds 6 reference points on the outer edge of the double ring. From those 6 points Throw works out how every pixel in the image maps to every millimetre of the board. It also measures the rings on your actual board (treble, double), because not every board is identical to the millimetre.

Calibration happens at the start, with the board empty and still. If someone knocks the phone mid-game, Throw notices and recalibrates as soon as things settle down.

Step 2: compare before and after

This is the heart of the system, and there's no AI involved at all. Throw keeps an image of the board from before your dart. When something moves (your dart arriving), it waits for everything to settle, a fraction of a second while the flight stops wobbling, then takes an after image.

It compares the two. Darts already in the board appear in both images, so they cancel out. What's left is the new dart, and only that. Before comparing, Throw lines the two images back up: the board and the phone always shift by a pixel or two on each impact, and without that step everything would look "changed".

Step 3: find the tip

Spotting the dart isn't enough. What matters is where the tip goes into the board, and a steel tip is about 2 mm wide, close to invisible. Throw finds it with geometry: it picks out the shape of the dart, works out its axis, then follows that axis to the point where the dart enters the board.

Step 4: score it

Once the tip is placed, it just has to be dropped onto the straightened board: how far from the centre (single, treble, double, bull?) and which segment (20, 1, 18...). It's pure calculation, the same position always gives the same result.

When you pull your darts out, Throw sees they're gone and confirms the visit. Next player.

It all runs on your phone

No image has to go to a server to be analysed: all the work happens on the phone. Throw is a web app you install straight from your browser, no app store needed, on iPhone and Android alike. The calibration model downloads once, the first time you start the camera, and after that the app works offline, in the garage or down the pub.

How well does it work?

To test it honestly, we filmed real play sessions with the phone off to the side, next to an Autodarts setup with three cameras acting as referee. Over those sessions (144 darts):

• 98% of darts were picked up;
• roughly 9 darts out of 10 landed in the same segment Autodarts gave. And the referee gets it wrong now and then too: checking the disagreements frame by frame, a few were on Autodarts' side.

For comparison, our first approach, which looked for the darts in a single photo at the end of the visit, topped out at 57% on the same footage.

The limits, honestly

One camera means one point of view, and some situations stay tricky:

• Bounce-outs: a dart that falls out of the board leaves nothing to see. Tag it as a bounce with a tap.
• Hidden darts: if the flight of a dart already in the board hides the new one, the camera can't make up what it can't see.
• On the wire: within a millimetre or two, the call can go the wrong way.
• A light tip on a light bed: sometimes it barely shows up at all.

In every one of these cases, fixing it is meant to be quick: tap the dart's slot, pick the right value, and the score is recalculated.

Now: online and on the big screen

Two new things are here:

• Online play against your friends, one on one, each of you at your own board with your own phone. No video: your opponent sees an image of your board with every dart, and every correction shows on their side. You need a (free) account to play, not to watch a public game.
• A screen for the living room or the bar: open throw.maz-ui.com/app/screen on a computer or a TV, then on the phone, in the game menu, tap Screen and type the code shown. The game your phone is scoring appears on the big screen, no account needed.

The phone keeps everything: every game stays playable on your own and offline, accounts and online play come on top, never as a requirement.