1. A method of displaying a 3-dimensional image, comprising: forming a first parallax image for a left eye and a second parallax image for a right eye, each of the first and second parallax images including a plurality of display images of an object at different depths; controlling a brightness level of each of the display images in each of the first and second parallax images to control a depth of each of the first and second parallax images; and displaying the first and second parallax images, wherein displaying the first and second parallax images comprises composing a first image for the left eye by combining the display images of the first parallax image and a second image for the right eye by combining the display images of the second parallax image, wherein the first and second parallax images are displayed apart from each other by a predetermined distance on a real display panel, wherein the displayed first parallax image corresponds to projections of a first virtual image of the object on a first virtual display panel and a second virtual image of the object on a second virtual display panel onto the real display panel, wherein the displayed second parallax image corresponds to projections of the first virtual image of the object on the first virtual display panel and the second virtual image of the object on the second virtual display panel onto the real display panel, wherein the first and second virtual display panels are positioned at different depths from the real display panel, wherein the display images for each of the left and right eyes comprise a first display image corresponding to the projection of the first virtual image of the object at a first depth of the different depths and a second display image corresponding to the projection of the second virtual image of the object at a second depth of the different depths, said second depth being deeper than the first depth, and wherein forming the first parallax image for the left eye and the second parallax image for the right eye comprises forming the first and second display images for the left eye such that the second display image is positioned left from the first display image by a predetermined distance and forming the first and second display images for the right eye such that the second display image is positioned right from the first display image by the predetermined distance, or forming the first parallax image for the left eye and the second parallax image for the right eye comprises forming the first and second display images for the left eye such that the second display image is positioned right from the first display image by a predetermined distance and forming the first and second display images for the right eye such that the second display image is positioned left from the first display image by the predetermined distance.