An edge function is computed from two vertex coordinates given by a rendering target line. Gradient determination is then performed on the edge function, and the functions representing two shift lines that are formed by translating the rendering target line in the y-axis direction or x-axis direction depending on the angle formed by the rendering target line and the x-axis of the rendering plane coordinate system by 0.5d and −0.5d will be computed, where d denotes the width of one pixel. Then, the number of subpixels included in a parallelogram, which has four points of the starting points and ending points of the two shift lines as vertices, is acquired so as to determine the pixel value of each pixel based on the number.