1 · Video source
Phones with several rear lenses list each one under Camera. Many phones crop the picture in 60 fps mode, so “Widest view” is the default. Hold the phone sideways (landscape) for the widest horizontal view, and use a tripod: the camera must not move after calibration.
Slow-mo factor applies to video files only: a 240 fps clip that plays back at 30 fps → 8.
2 · Calibration
Click both ends of a reference line on the ground, in the plane of the flight (e.g. a 5 m tape laid along the throwing direction). The line also defines “horizontal” and ground level.
3 · The ball
Put the ball in the catapult cup. The app looks for it by itself (AI + round-object finder) and asks “Is this the ball?”; answer No to see the next candidate, or simply tap the ball on the picture. On Yes it learns the ball's colour (any bright colour), size and position and locks onto it. Redo it whenever the ball or the light changes. The mask shows exactly what is detected (magenta).
Use a detection zone to ignore orange shirts, cones or the crowd outside the flight area.