1. A three dimensional graphics processing apparatus comprising: a polygon edge categorizing section for categorizing each polygon edge included in at least one three dimensional polygon of a three dimensional graphics image as being at least a left edge or a right edge in accordance with an inclination of the polygon edge being categorized in a two dimensional XY-coordinate plane; 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 from among drawing points corresponding to a first intersection point between the polygon edge being categorized and a first one of adjacent scanning lines and a second intersection point between the polygon edge being categorized and a second one of the 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 first and second intersection points, with the area ratio calculated by the area ratio calculating section being based on the respective coordinates of the drawing points calculated by the coordinate calculating section; wherein each categorized polygon edge has a starting vertex and an ending vertex, and the coordinate calculating section includes: an X-axis direction distance obtaining section for determining which of (i) rounding-off of a distance in the X-axis direction from the starting vertex of the categorized polygon edge to the intersection point between the categorized polygon edge and a scanning line by adding 0.5 to the distance and truncating a resultant decimal part and (ii) rounding-down of the distance in the X-axis direction from the starting vertex of the categorized polygon edge to the intersection point between the categorized polygon edge and the scanning line by truncating the decimal part of the distance based on the inclination of the categorized polygon edge, wherein rounding-off is performed when the inclination is smaller than or equal to 1 and rounding-down is performed when the inclination is larger than 1, so as to obtain an integer distance in the X-axis direction from the starting vertex of the categorized polygon edge to the intersection point between the categorized polygon edge and a current or a next scanning line.