Fig. S2.

(A) Tracked 2D flagellum in the focal plane of the microscope in the head-centered comoving frame $\left(\tilde{x},\tilde{y}\right)$, in which the $\tilde{x}$ axis is aligned with the mean swimming direction; s denotes the arc length, and n denotes the line normal to the flagellum. (Scale bar, 10 μm.) (B) Intensity cross section along the flagellum. The dashed white line, $n=0$, denotes the position of the flagellum centerline. Whenever the centerline passes through a dark (bright) region, the z coordinate is above (below) the focal plane, indicated in green (red) shaded regions. Two sample intensity profiles $S1$ and $S2$ are shown for illustration. (C) The width of the intensity maxima determines the z displacement, allowing for the 3D reconstruction of the flagellum (Movie S7).