Information density of an interface examines the compactness of an interface in terms of the amount of information. The information density metric is calculated as the quantity of visible node instances containing information divided by the screen space area of the whole interface in pixels. In order to compare different interfaces, one has to count instances of the same node type (e.g. web pages). While the argument here is not to encourage designs where the screen is always filled, I claim that efficient use of the screen space is crucial as it has the effect of expanding the short-term memory of users. Screen space must be cleverly used for information related to user's task making maximum use of the screen real estate.
If we examine the WebBook and the Web Forager interface, space is typically used for the decoration of the desk and the shelves. Space is also left unused in the air. However, the book node increases the information density considerably, where many pages are compactly stored using only a single page size. In the vtwm window manager, a window many contain multiple pages, where each page is accessible through the history, the bookmarks, etc. Besides windows may also overlap, further increasing the information density. The overview node provides users access to multiple desktops, thus to multiple pages. Thus, it also increases information density. In the Elastic Windows interface due to the elastic nature of the layout more pages can be compacted into a small area. However, since they are organized in a non-overlapping manner, the information density is not high. However, the Elastic Windows interface allows users to pack hierarchies of windows into bars, which improves the information density considerably, since they are of fixed size and may contain arbitrary number of pages.