Each Displayble object contains a display list, as well as information about the item to be drawn such as whether to display or hide the object, it's current transformation (how much to rotate, scale, and translate the object), whether it is a 2D or a 3D object, whether it is fixed (set to ignore any requests to scale, translate, or rotate the object), and a list of children. A Displayable registers with one or more Scene objects, so that the Scene then will contain a list of all the Displayable objects which should be drawn when requrested. The Scene is responsible for providing the physical memory storage for a display list, through a request by a Displayable. Each molecule and other graphical item in VMD is a Displayable subclass. These derived objects supply the methods to fill the object's display list with drawing commands for the item to be drawn, i.e., the commands to draw the points for each atom and the lines for each bond in a molecle.
A key feature of each Displayable is that it may contain any number of child Displayable objects, and may also be a child of some other parent Displayable. Each child may be individually translated, rotated, etc., and may be turned on or off individually; but operations to a parent such as a transformation typically affect all the children of that parent as well. For example, if a parent is currently being hidden, so will be all the children of that parent. Also, only Displayable objects which have no parent register with a Scene; all children of a parent are drawn properly when the parent is drawn, so it is only necessary for the very top-level parent to be stored in the Scene.