providing object design data in which a structural feature forming part of the physical object is represented by generating a parametric representation of the structural feature in the form of a line or surface, wherein the line or surface does not ever form part of a volumetric representation which encloses a volume of the physical object;

then slicing a section of the line or surface using an intermediate slicing layer that is provided between first and second physical build layers of the additive manufacturing apparatus, wherein slicing the section of the line or surface generates an intermediate layer point or line at an intersection of the section of the line or surface and the intermediate slicing layer, wherein the intermediate layer point or line is located between the first and second physical build layers;

projecting the intermediate layer point or line to a projected build layer point or line that lies within a physical build layer of the additive manufacturing apparatus; and

using the projected build layer point or line to provide tool path data for that physical build layer.