In an example, a method includes receiving, at least one processor, object model data representing at least a portion of an object that is to be generated by an additive manufacturing apparatus by fusing build material within a fabrication chamber, wherein the object model data comprises a mesh model comprising data describing a surface of the object. A geometrical compensation vector may be determined for the object model data, the geometrical compensation vector having a first component applying to a first axis of the fabrication chamber and a second component applying to a second axis of the fabrication chamber. A geometrical compensation to apply to at least one location on the surface of the object may be determined by determining a product of the geometrical compensation vector and a vector indicative of the normal of the object surface at the location and the determined geometrical compensation may be applied to the object model data to generate modified object model data.