Making scrollable backgrounds

Scrolling backgrounds are formed by a set of sprites positioned directly next to each other. A full-size sprite is only 16 pixels wide, so 20 are needed to fill the active display horizontally, with a 21st one used to fill the gap that would be present on an edge when scrolling by less than 16 pixels at a time. The sticky bit can be used to make this simpler and avoid the overhead of updating the position of so many sprites.

It is possible to take advantage of the 512 pixel wrapping property of sprite positions to set up a layer made of 32 sprites, and update the tiles of individual sprites just before they appear on screen. However all 32 sprites would count in the sprites-per-line limit of 96, which is very wasteful.

A better method is to "rotate" a set of 21 sprites and dynamically chaining and unchaining them.

Demos

Interactive demos with source can be found here.

(todo: clean it up, make scroll3, finish documenting it here)