1. An image generating method for rendering a three-dimensional object viewed from a predetermined viewpoint by generating an image of the three-dimensional object and writing color information on the image generated in a rendering buffer, the method comprising: generating at least one of retouched image of the three-dimensional image by arranging a plurality of brush images so as to superpose a part of the plurality of brush images on one another part of the plurality of brush images within a rendering region for the three-dimensional object, the rendering region on which the three-dimensional object is projected on the basis of the predetermined viewpoint; generating a projection image by projecting the three-dimensional object on the basis of the predetermined viewpoint; and rendering the image of the three-dimensional object so as to reflect color information of the projection image at a part at which the retouched image is transparent by synthesizing the retouched image with the projection image.