Visualization of concurrent markup
From trees to graphs, from 2D to 3D
The present paper deals with the visualization of concurrent markup. An initial discussion of the underlying model of XML instances demonstrates that valid XML exceeds the expressive power of trees. While some challenging features of concurrent markup, like overlaps, can be captured by minimally extended trees, there are other phenomena which can be adequately expressed in XML using constructs which instantiate advanced graph structures (e.g. discontinuous elements or repetitive structures).
On the basis of two representation formats for concurrent markup, XStandoff and xLMNL, two distinct approaches towards its visualization are presented. The first method has been implemented in XSLT as an SVG-based 2D visualization strategy. Although it can be shown that this first approach provides an adequate (though not optimal) solution to overlapping structures, it is not capable of illustrating enhanced graph-based phenomena like the ones mentioned above. Therefore, some remarks about possible 3D visualizations are made which show how the adding of another dimension could contribute to the appropriately expressive visualization of concurrent markup. In addition, a prototypic implementation based on XSLT and X3D is discussed as first step towards a three-dimensional illustration.