Electronic holography is the ideal 3D image system. However,, it requires huge amount of data. The image size is small and viewing-zone angle is narrow because the required pixel size must be comparable to light wave length. To overcome these problems,, we have investigated hologram generation method from depth images,, which reduces the amount of data for storage or transmission,, a spatio-temporal multiplexing display method,, which enables a wide viewing-zone-angle for reconstructed holographic images together with full-colorization,, and a 3D image projection method to enlarge image size. Recent progress in these technologies are reviewed and discussed.