Inspired by EMSL’s holiday cards, Grathio wrote up his steps for creating edge-lit displays for general usage covering an easy method for multi-image displays –
Lets say we’re making an electronic Tic-Tac-Toe game. Ideally we want either an X or an O in the same place. To do this we make and X and an O with a separate LED for each one. Since each piece is essentially transparent when off, we can put one behind the other to have a display that can show an X or O in the same space, depending on which LED we light. You can also use the same effect to add more than one color to the same display or animate parts of an image like some animated neon signs.
Check out the Edge Lit Displays instructable for more details.