How golf shot tracers work, and how to add one to your videos for free

How TV does it

The first tracer on a golf broadcast was Protracer in 2006, which picks the ball out of each frame with its own cameras (Topgolf bought it in 2016 and it's Toptracer now, the same thing that's in a lot of driving range bays). The PGA Tour went with Trackman from 2022, which is radar, so it measures the ball's actual speed and direction and doesn't need to see it at all. Both of these are hardware on the course, and neither is something you can point at a video you already filmed.

How it gets onto everyone else's videos

There are two ways as far as we can tell. The first is an app that looks for the ball for you, and the best known is Shot Tracer ($5.99, and the live tracer and editor are $9.99 a month or $59.99 a year on top), with a desktop version, Shot Tracer Pro, at $14.99 a month or $89.99 a year that handles a moving camera. V1 Golf has one inside a paid upgrade, and the Toptracer Range app is free but only works in a bay that has Toptracer. There are free ones too, Golf Ball Tracer is free with its logo on the export, and you tap where the ball starts, the top and where it lands and it draws a smooth arc through them.

Finding a white ball that's two pixels wide against a bright sky is really hard though, and the reviews say so. Shot Tracer's own tutorial says a tripod is essential, one reviewer spends 5 to 10 minutes fixing about 75% of shots by hand anyway, and one Shot Tracer Pro review says auto-detection worked 4% of the time over 600 traces (which is one person, but it's not a great sign).

The second way is by hand in an editor, After Effects or Motion or CapCut, where you draw the path as a mask or a stroke and keyframe it so it draws on over the shot. It works and it's how you get full control, but it's pretty slow, and the result usually looks a bit off even when the path is right, which we think comes down to timing.

Why a drawn arc looks wrong

A golf ball doesn't really fly a parabola. It leaves a driver at about 167 mph and 10.9° (Trackman's tour averages), and drag starts slowing it right away while backspin gives it lift, so it keeps climbing longer than it should, and then once it's slow the lift is gone and it comes down at 38°, much steeper than it went up. So the shape is lopsided, and more importantly the ball is fast at the start and slow at the end.

That second part is probably what your eye picks up on. A line that grows at an even speed along the right path is still ahead of the ball near the tee and behind it on the way down, and when you're watching the ball and the line at the same time it just looks drawn on, even if you can't really say why.

What Corsai does instead

We don't try to find the ball, you tell us where it is. You tag the strike, tag the ball a couple of times in the air, and tag where it lands, and each tag is a spot on the screen and a moment in the video. Then we fit a small simulation of the flight to those tags, with launch angle, drag, lift from backspin and sidespin, seen through a camera standing behind the golfer, six numbers in all, solved with Levenberg-Marquardt (the standard way to fit a curve to points, nothing exotic).

The important bit is that the fit is scored on where the ball was and when. The strike and the landing fix the flight time exactly, so the simulation's own clock becomes the line's clock, and the head of the line is where the simulated ball is at that frame, which is how it stays on the ball the whole way. It's worth being honest that the model works in its own made-up units and in pixels, so it knows the shape and timing of your shot and not your ball speed (it's a tracer, not a launch monitor). The same model with its usual driver settings, launched at 10.9°, lands at 38° like Trackman's average does, which we didn't tune it to do and were pretty happy about.

More tags pin it down more, and the tag high up matters most. On synthetic test shots (a simulated 1080×1920 phone video where we know exactly where the ball is), the worst miss anywhere along the line, with the landing marked, looks like this.

Before you mark the landing, the fit also guesses where the ball is going and draws it as a dashed line, which with one tag is mostly guesswork (off by 75 to 110 px on the landing) and with three is usually close, at least on the test shots. And if you zoom in to follow the ball while filming, the tracer reads the camera first, frame by frame with a tracker of our own, and pins your tags to the scene rather than the screen, so the line rides through the zoom. On a real handheld clip with about 16x of zoom the browser's track was off by 18 to 35 px at 1080 wide (2 to 3% of the frame), where Apple's built-in alignment was off by 400 to 500 px, which is why we wrote our own. It does lose its place on plain blue sky with nothing to track, and it hides the line on those frames rather than guess.

Adding one for free

It exports an MP4 at the clip's own frame rate, with sound where the browser can encode it, and a small corsai mark and the page's address in the corner (that's the price of it being free). It works in Chrome, Edge, Firefox and Safari, on an iPhone too. If you record your rounds on an Apple Watch, the same tracer is in the Corsai iPhone app's golf stories, along with a hole number on every shot.