Examples of the present disclosure relate to methods for processing image data comprising data corresponding to a three-dimensional object. One such method comprises determining a plurality of polygons, the polygons being representative of the three-dimensional object, wherein each polygon has associated geometric information. At least one polygon group is formed from the plurality of polygons, said polygon group comprising polygons with correlated geometric information. At least one geometric primitive is determined for said polygon group, the geometric primitive corresponding to polygons of said polygon group. Data is output, representing said geometric primitive as at least a part of a representation of the three-dimensional object.