In an example, a method for generating control data for production of a three-dimensional object is described. A model of the three-dimensional object is obtained as a array of voxels, and it is determined for each voxel whether that voxel comprises part of a first or a second sub-object of the three-dimensional object. Each first sub-object voxel is mapped to a volume coverage representation defining print material data for that voxel. The second sub-object voxels are mapped to a volume coverage representation defining common print material data for the voxels of second sub-object. Control data for printing the first sub-object is generated from the print material data for that voxel common print material data for the Control data for printing the second sub-object is generated according to the volume coverage representation for the second sub-object.