DisplayDevice is the base class for objects which do the actual rendering of a Scene. Each derived class of this parent must provide versions of virtual functions which prepare the device for drawing (i.e., clear a screen or open a file), render a list of drawing commands, and update the display after drawing, among many other functions. This class is where library-specific drawing commands are encapsulated, for example there exists a GLDisplayDevice subclass which is used to draw images using the GL graphical library from Silicon Graphics, Inc. A DisplayDevice can also be defined which renders a Scene to a file instead of a monitor, i.e., to a postscript file, a raytracing program input script, or a bitmap image file.