1. A display device comprising: a light blocking panel having an adjustable transparency and configured to block light or allow light to pass therethrough; a transparent display panel disposed on the light blocking panel and configured to output a display image; a three-dimensional (3D) panel disposed on the transparent display panel and configured to generate viewpoint disparity to generate a 3D effect in at least a portion of the display image, wherein the light blocking panel and the 3D panel are each organized into a plurality of blocks corresponding one or more pixels; a mask generating unit configured to generate a first mask from a difference image map generated from a left eye image and a right eye image that indicates whether or not viewpoint disparity is generated in each block of the 3D panel, and to generate a second mask from the first mask that indicates whether each block of the light blocking panel is transparent or opaque; and an image processing unit configured to generate the display image from the left eye image, the right eye image and the first mask, wherein the display image is divided into a first region and a second region on the basis of the first mask, wherein the first region includes an interlaced image generated by alternately arranging left eye image pixels and right eye image pixels, and the second region includes a 2D image generated from one or both of the left eye image and the right eye image.