Software that is able to cut out or "knock out" one part of an image. An image editor can be used, but requires that the user trace the object with extreme precision, which may be impossible if it is very complicated. With masking software, the user still has to trace a line around the area, but it can be done a lot more casually, letting the algorithms in the software figure out every last pixel.
Which Is the Original?
With Ultimatte KnockOut Software (UKO), it is hard to tell the original background from the phony one, because the software cuts it out with extreme precision. The alpha channel (bottom right) shows the mask that was created. (Image courtesy of Ultimatte Corporation, www.ultimatte.com)