Move the phone and the shape stays put — sideways, up and down, and now in depth as well. Pull the phone away from your face and it grows; bring it closer and it shrinks.
That direction is the correct one, not a preference. A ray from your eye through a fixed object crosses the screen plane at r = (e/D)·R, so the drawn size scales with the eye‑to‑screen distance. At a 35 cm viewing distance, 5 cm of movement is a 14 % size change.
Turn and tilt the phone for a few seconds after starting; the ring at the centre fills as the rotation model is learned. Tap anywhere to recentre.
Scale is 1 − z / e. Shorter viewing distance means a stronger effect for the same movement.
Corrects 57 % of a step within 100 ms.
Rectangle: — × — px.
Settled offset from a 0.03 m/s² residual: 10 mm.
Roll axis: detecting.