Disclosed is a computer-implemented method for determining a boundary representation of the result of a two-axis 3D printing process. The method comprises obtaining a stack of planes, each couple of consecutive planes corresponding to a respective slice of the result of the 3D printing process. The method also comprises, for each couple of consecutive planes: obtaining one or more respective tool trajectories, determining a respective 2D contour corresponding to the one or more tool trajectories. The method also comprises determining 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 the boundary representation with the extrusions and portions of the planes. The method improves 3D printing.