In image alpha-compositing, the Duff–Porter operators are a standard set of 12 rules for determining the RGB values of each output pixel as a function of the RGB values of two corresponding input pixels. In this Demonstration an image of a spiral staircase (the "object") is composited with a background image of a tiger (the "image"). The default object "over" image compositing method corresponds to the usual application of alpha blending to semi-transparent foreground objects. Different compositing methods produce other interesting effects.