1. A method of determining a contribution of an object to an intensity value of a current pixel that is partially covered by the object, the method comprising: (a) calculating an entry location and exit location for an edge of the object in the current pixel, the entry and exit locations being located on boundaries of the current pixel with perpendicular orientations; (b) determining an intensity value for a neighbouring pixel located adjacent to the current pixel, the neighbouring pixel being selected from adjacently located pixels based on the calculated entry and exit locations; and (c) determining an intensity value of the current pixel using the determined intensity value of the neighbouring pixel.