In one example, a representation of a volume of a part to be printed by an additive manufacturing system is generated, where the representation depicts a plurality of voxels. A plurality of slices of the representation are generated. Each slice relates to a plurality of voxels within a first plane. A first process is performed in respect of a slice of the plurality of slices, wherein in the first process at least one voxel that has a predetermined colour and is located on a predetermined surface of the part is identified and a data file is updated to include data representative of the identified at least one voxel. The first process is repeated in respect of another slice. A three-dimensional preview of the part to be printed is generated based on the data file.