Methods and systems, including medium-encoded computer program products, for generating internal structures usable in additive manufacturing include: obtaining a three-dimensional (3D) model of an object to be created by a 3D printer; determining a first slice of the 3D model; and generating first slice data that represents a portion of an internal structure in the first slice of the 3D model of the object, the slice data being usable for fabricating the first slice of the object during a 3D printing process, wherein the portion of the internal structure (i) includes cell structures corresponding to a respective point of a plurality of points with corresponding locations within the interior region, wherein each of the cell structures include an empty space inside the cell structure and (ii) includes a determined quantity of the cell structures based on an analysis of the 3D model.