A scroller is a game in which the terrain is bigger than the actual screen. Instead of moving the character, you move the terrain behind him the opposite way. Many Scratchers use ScrollX to keep track of the "real" amount of distance they have traveled.
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Here is a great article about scrolling if you want detailed information about how to scroll, and different ways of scrolling.
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It's where the terrains are bigger than 480 pixels (the width of the screen). To get around this, some people use a method called scrolling, which is where it looks like the character sprite is moving, but it's actually just the terrains moving in the opposite direction
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