1. An image generation system which generates an image visible from a virtual camera in an object space, the image generation system comprising: an alpha-value synthesis section which synthesizes a first alpha value and an alpha-nonuniformity setting alpha-value into a second alpha value, the first alpha value corresponding to a Z-value of each pixel in an original image, and the alpha-nonuniformity setting alpha-value of each pixel being nonuniform in a virtual plane; and a color synthesis section which synthesizes a color of each pixel in the original image and a given color, based on the second alpha value.