@Venbha, the image is created using program code. Four different colors are drawn onto the four quadrants of a blank square image and the transparency is set to half. All actors use the same image at any one time.

@Yogibaer, nothing is special about class configurations -- and the Scroller class is not directly linked to the World or Actor class, but is a "help class" in itself extending the Object class (the root of ALL classes). A subclass of world that is to scroll will create a Scroller object and tell it when and how much to scroll. That is basically it.