The light coming out of the scratch-hologram must be coherent. That means, the scratches must be mirrors (like smooth grooves, or bent silver wires, shiny curved spiderwebs.) Normal raster-displays emit incoherent light, and their images are entirely located on the display surface. But make your display out of billions of tiny mirrors, then you can create images which float deep inside the surface. (Or hang in the air, outside the surface.)