a) Face-on view of a disc after an encounter with m₁₂ = 1 and r_peri = 2 r_init. For clarity, the particle angles have been decorrelated to destroy the spiral arms. The vertical lines mark the disc diameter (=2r_disc) as obtained with our disc size definition. b) Corresponding initial (thin) and final (thick) particle (solid) and mass (dashed) surface density distribution (all lines are smoothed). The vertical line shows the final disc size as obtained with our disc size definition.