At least a portion of a 3-D object is converted into a format suitable for printing. A material volume coverage vector for each voxel of a 3-D bit map of a 3-D object is determined. The 3-D bit map comprises a plurality of voxels, each voxel located at a unique 3-D location. A rectangular cuboid comprising an M×N×L array of voxel locations enclosing at least a portion of the 3-D object is determined. A halftone threshold matrix comprising an array of threshold values is provided. The halftone threshold matrix is converted to provide an M×N×L 3-D array of threshold values. Each material volume coverage vector of the at least a portion is compared with each threshold value at corresponding 3-D locations to select a printable voxel at each 3-D location to convert the at least a portion of the 3-D object into a format suitable for printing.