Sketch of the procedure we used to obtain the collisional cross section. In the comoving frame, the planetesimal resides at the origin (y = 0) and pebbles start at a distance y_S far enough to be unperturbed by the gravity of the planetesimal. Integrations are first carried out in the x-direction, and the impact parameter b_coll,x is determined from the first and last trajectory that hits the planetesimal (x_min and x_max). Starting at the average of these two coordinates x₀, the integrations are extended to the z-direction to obtain the vertical impact parameter b_coll,z.