The 3-D motion of our microswimmers is tracked using an Extremely Randomised Decision Tree ensemble model (ERTs), as outlined in further detail in. Briefly, labelled data to train the model was first obtained from 280 µL dilute aqueous suspensions of the TiO 2 -Fe 2 O 3 -SiO 2 Janus particles in a flow-through cell (cell 137-QS; Hellma Analytics) with a light path length of 1 mm. Z-stacks of the sedimented Janus particles were taken, and the Z-slices serve as the labels for the image data. Particles were imaged on an inverted microscope (Nikon Eclipse Ti2e) under Köhler illumination with white light using a 40× objective (CFI S Plan Fluor ELWD 40XC) with adjustable collar (set to 1 mm), and Z-stacks were taken with an exposure time of 30 ms using a Hamamatsu C14440-20UP digital camera. To simulate the conditions of swimming experiments, the particles were also illuminated with UV (340 nm, without fuel), using a Lumencor SPECTRA X light engine as the excitation source through the objective (epifluorescence).