DRS on Your Phone: How Ball Tracking Reviews an LBW

Every club cricketer has had the argument: that was plumb against that was going down leg. On television the argument ends when the ball-tracking replay appears. At the nets, at training or in a club game, it usually never ends at all.

The good news is that the logic behind a DRS ball-tracking review is not magic, and it no longer needs a stadium full of cameras. This post explains exactly how an LBW review is reconstructed, and how you can run the same review on your own deliveries with nothing more than a phone.

What an LBW review actually asks

The Laws of Cricket set out when a batter is out leg before wicket in Law 36. A ball-tracking review breaks that Law down into three questions, always asked in the same order:

Pitching. Where did the ball land? If it pitched outside the line of leg stump, the batter cannot be out LBW, no matter what happens next.
Impact. Where did the ball hit the pad? If it struck the batter outside the line of off stump while they were playing a shot, it is not out.
Wickets. If the pad had not been in the way, would the ball have gone on to hit the stumps?

Only when all three come out in the bowler's favour is the decision out. That is why a review graphic always shows three verdicts in a row, and why a ball that would have smashed middle stump can still be not out because it pitched outside leg.

The Law also tells the tracker what to assume about the third question: the ball is taken to carry on along its path after it hits the pad, as if the pad were not there. That assumption is exactly what a trajectory model is built to answer.

How ball tracking answers each question

A ball-tracking system does not "see" the ball hit the stumps, because it never does. It measures the part of the flight it can see, fits a physical path to it, and projects that path forward.

Pitching is measured directly. The tracker finds the ball in every frame, sees the bounce, and places the bounce point on the pitch relative to the stumps.
Impact is also measured: the point where the ball stops following its path, placed against the line of the stumps.
Wickets is the projection. The flight before impact (its speed, its angle, and how it moved after pitching) is extended past the pad to the stumps, and the system reports whether that path hits them.

The key ingredient in all three is calibration. To know whether a ball pitched in line with the stumps, the software has to know where the stumps are in real, physical space, not just on the screen. It uses the pitch itself as the ruler: under Law 6 the stumps stand 22 yards apart, so once the software finds both sets of stumps and the creases in the frame, every pixel has a real-world position.

Why this used to need a broadcast truck

Television ball tracking uses several high-speed cameras placed around the ground, each seeing the ball from a different angle. Combining the views pins the ball's position down very precisely, which matters when a decision at international level turns on millimetres.

A single phone behind the bowler's arm sees the ball from one angle, so it has to work harder: it relies on the calibrated pitch, a physical model of the ball's flight, and a clean view of the whole pitch. What it gives up in millimetre precision it gains in something the broadcast system can never offer a club cricketer: it is there every time you bowl.

Reviewing your own deliveries with Zoota

Zoota brings this three-part review to a phone. You prop your phone up behind the bowler's arm, facing down the pitch, and record your session. For every delivery Zoota finds the release, tracks the ball, calibrates itself from the stumps in frame, and fits the flight path.

For any ball that reaches the batter, Zoota's DRS review reconstructs pitching, impact and wickets, and lets you watch the projected path past the pad in a 3D replay of the delivery. The same tracked flight also gives you:

Speed from release to bounce, calibrated to the pitch.
Line and length on a pitch map, next to every other ball you bowled.
Swing and spin, measured per ball.

For bowlers, that turns an argument into a training tool. A spinner can see how often they are pitching in line and hitting the stumps, which is the whole job of a wicket-taking stock ball. A seamer can see whether their natural angle takes the ball down leg, which no amount of feel will tell them.

For batters, the same review answers the question they care about: was I actually in trouble, or was that going over?

Getting a clean review

The review is only as good as the view. Three things matter most:

Keep the full pitch and both sets of stumps in frame. The stumps are the ruler. If one set is cut off, the tracker cannot place the ball against the line of the stumps.
Mount the phone steadily behind the bowler's arm. A tripod or a net-fence mount keeps the calibration stable for the whole session.
Use good, even light and the highest frame rate your phone offers. More frames means more measured positions to fit, and a tighter projection.

A word of honesty about projections

Any ball-tracking verdict on the wickets question is a prediction, because the ball never actually reached the stumps. That is true on television too, which is why professional DRS keeps an "umpire's call" margin for balls that only just clip the stumps.

Treat a close projection the same way: as probably hitting rather than a certainty. Where phone tracking really shines is in the pattern across a session. If eight of your ten stock balls would have hit the stumps, that is a signal no single close call can argue with.

The short version

An LBW review asks three things in order: did it pitch in line, did it hit in line, and would it have hit the stumps. Ball tracking measures the first two directly and projects the third from the ball's calibrated flight. You no longer need broadcast cameras for that: a phone behind the bowler's arm, with the whole pitch in frame, can reconstruct pitching, impact and wickets for every ball you bowl.

Zoota gives you a DRS-style review, a 3D replay, speed, line and length for every delivery, from a single phone recording. It's free to try on Android, with 100 tracked balls a month on the free plan.