The invention notably relates to a computer-implemented method for determining a B-Rep of the result of a two-axis 3D printing process, the method comprising: providing (S10) a stack of planes, each couple of consecutive planes corresponding to a respective slice of the result of the 3D printing process; for (S20) each couple of consecutive planes: providing (S210) one or more respective tool trajectories; determining a respective 2D contour corresponding to the one or more tool trajectories; determining (S230) a respective extrusion of the respective 2D contour bounded by a second plane of the couple and a first plane of the couple; and forming (S30) the B-Rep with the extrusions and portions of the planes. The method improves 3D printing.