9.2 IGD - Hierarchical Display of Hypertext Structure
Steven Feiner [Fei88] describes a hypertext system that is based on a strict tree structure for the organization of the nodes. His system IGD (Interactive Graphical Documents) supports the creation and the display of arbitrary keyworded graph structures embedded in a tree hierarchy. Like most of the systems that are currently available, IGD does not support the conversion of existing documents into a tree structure. Rather an aspiring author is expected to write new tree-structured documents using IGD. IGD supports the creation of tree hierarchies, but the text has to be put manually into the hypertext nodes.
To make out the best tree representation for IGD, Feiner [Fei88] investigated the respective advantages and disadvantages of different ways to display tree structures, as identified by Knuth [Knu73]. Knuth describes three standard graphical notations for displaying trees: graph trees, indentation, and nested set notation. Figure I.35 contains the popular graph tree notation. Feiner thinks that this representation gets too tangled and messed up as soon as the number of nodes and hierarchy levels becomes too large.
Figure I.35 Graph tree notation
Figure I.36 displays the same graph in the indentation view. This eliminates the need for explicit tree arcs, and it also provides a clearer distinction between the hierarchical and the arbitrary graph structure. However, the indentation view suggests a sequential ordering of the children of each node.
Figure I.36 Indentation graph notation
The nested set notation expresses the tree structure through spatial containment by nesting each node within its parent node (figure I.37). Steven Feiner considers this representation superior to the two aforementioned approaches for the purpose of editing the tree structure. He claims that this method offers the best visually distinct technique for showing hierarchy and graph structure. Also the only restriction for a node's placement is the arrangement within its parent's boundaries.
Figure I.37 Nested set graph notation
IGD therefore uses a variant of the nested set graph notation. To improve the representation of information by information hiding, IGD employs three techniques:
- subtree detail suppression:
- It is possible to represent selected subchapters only by their bounding box. In figure I.37 chapter "F" could be represented only by the box without showing the two nodes contained within.
- subtree display selection:
- The user can walk up or down in the tree hierarchy and fill each subchapter with as much enclosed information as desired.
- link inheritance:
- To suppress links that have no importance in a local context, the concept of link inheritance is introduced. It allows one to show only links connecting nodes with the same ancestor. For example, assuming that the user is focusing on chapter "D" in figure I.37, the link between the left node in "C" and the one node in "E" would be suppressed.