HamsterEngine is a horizontal scrolling engine made for people who are new to scrolling in Scratch. In this case, scrolling means that the ground sprites (flats) move to make it look as if the hamster sprite is walking. HamsterEngine uses flats and scenes. Flats are sprites with 1-screen-wide (480 pixels) costumes. A scene can be thought of as the whole set of flats set side-by-side, and wearing costumes that paint the ground for the scene. A scene is really just a number that lets the flats choose the right costume. This approach to scrolling is a little complicated, but is meant to be flexible. The main character sprite (Hamster) sets the scrolling variable, xscroll, and all the moving sprites base their x-position on that variable. In the HamsterEngine example, there are 3 flats, and 3 scenes. Each flat calculates its "desired" x-position, and if it is partly or entirely on-screen, it also calculates the scene number, and chooses the appropriate costume. If it is off-screen, it hides. There are also background cloud sprites that base their movement on xscroll. Having them move at a slower rate makes a nice depth effect. Scratch on!
