In the games such as Defender (1980 arcade) and Defcon, the player can move/scroll in one direction infinitely because the level/terrain simply wraps over and over again(like if it was wrapped around the side of a cylinder).
I am more interested in the wrapping mechanics of Defcon which wrap(on the x-axis) and zoom the entire level. Here is a link to view the game.
To make images(such as a background) wrap is it a per-pixel approach? Is there a function? What methods are there to achieve the circular wrapping of a level?