Typically, as the amount of information in an interface increases its space requirements increase. The degree of dependence of the space requirements determines the scalability of an interface. A scalable interface provides a presentation hierarchy where the increases in the amount of information are matched by higher-level presentation entities, thus the overall space requirements are kept at a reasonable level. For example, in the desktop interface a window can be transformed into an icon, which can contain many other icons. Thus, the higher-level node (e.g. window) in the presentation hierarchy functions as a graphical abstraction mechanism. In practice, thus composite nodes have the effect of lessening the space requirements by representing group of node instances as a single visual primitive. Thus, the depth of the presentation hierarchy has a significant effect on the scalability.
In order to determine the total space usage of an interface, the space requirement (S) for each visible node instance in its presentation hierarchy must added. However, depending on the layout function, increase in the space requirements may not be directly proportional to an increase in the amount of information. For example, the layout function of the desktop node allows windows to overlap, thus the total space usage is less than the addition of the space requirements of each individual window. For that reason layout functions are of primary importance in examining scalability.
Let's examine scalability of the WebBook and the Web Forager, the vtwm, and the Elastic Windows interfaces by examining the depth of their presentation hierarchy and their layout functions. As in capacity metric, web pages are considered as the unit piece of information.
In the WebBook and the Web Forager interface, the layout function of the book composite node allows as many pages to be collected into a single book. The book is of a single page size when closed and two-page size when open. Thus, effectively the layout function of the book node allows the total space requirements to be independent of the increase in the quantity of the pages it contains. However, this is not true for the desk, the shelf, and the air composite nodes, since their layout functions yield typically a directly proportional increase in the space requirements. The depth of the presentation hierarchy for the WebBook and the Web Forager interface is 3 starting from the page node as the unit of information.
The layout function in the vtwm interface allows arbitrary overlaps. Thus, the space requirements may increase less than linearly, though visibility of each page decreases. The depth of the presentation hierarchy is 4, thus providing some scalability.
The layout function for the window node in the Elastic Windows interface, allows arbitrary number of non-overlapping pages and windows to be nested in a window instance due to the elastic nature of the layout. The space requirement of a window instance is thus independent of the amount of information (e.g. page and window), though visibility of each page decreases. The layout function of the window node allows packing a window into a vertical bar, thus abstracting all pages and windows it contains as a single visual entity and saving screen space. The depth of the presentation hierarchy of the Elastic Window interface is unlimited, contributing to the scalability to a large degree.