Regions 150 and 152 on a texture image plane 140 that correspond individually to pixels 146 and 148 representing an image on a texture image mapping target surface 144 in a screen plane 142 on which to define a display image are determined using predetermined transformation equations. The number of sampling points and their distribution are determined for the determined region in accordance with its shape and its size. Texel values in the determined region are then sampled. An average of the sampled values is acquired as the value of each of the original pixels 146 and 148.