a computer - controlled apparatus and method for fabricating three - dimensional articles in layerwise fashion is disclosed . upon dispensing a layer of a fusible powder , a laser irradiates selected locations of that layer to fuse the powder into a cross - section of the article to be formed in that layer , such that the fused cross - sections fuse together into the article . the laser is controlled in a raster scan fashion across the selected locations of the powder layer . the raster scan lines are defined , for each cross - section , to achieve an optimal fill scan time . the optimal fill scan time is determined , by the computer estimating the fill scan time by rotating the cross - section over a plurality of rotational angles , and estimating the fill scan time for each of the rotated cross - sections for at least a sample of the fill scans necessary to form the article . the actual fill scan vectors to be used in selective laser sintering of the article are rotated , from a coordinate axis at the target plane , according to the rotation of the cross - section providing the lowest estimated fill scan time .