1. A three dimensional graphics processing apparatus comprising: a polygon edge categorizing section for categorizing each of polygon edges included in a three dimensional polygon in accordance with an inclination of the polygon edge in the two dimensional XY-coordinate plane, the polygon edges being categorized into left edges and right edges; a coordinate calculating section for calculating coordinates of a drawing point having the smallest X-coordinate and coordinates of a drawing point having the largest X-coordinate, among drawing points corresponding to the respective intersection points between the polygon edge categorized by the polygon edge categorizing section and adjacent scanning lines; and an area ratio calculating section for calculating an area ratio of an internal region of the polygon occupied within a pixel of a predetermined unit region corresponding to each of the intersection points, based on the respective coordinates of the drawing points calculated by the coordinate calculating section.