Christoph Daniel Schulze

Drawing layered graphs with port constraints

Complex software systems are often modeled using data flow diagrams, in which nodes are connected to each other through dedicated connection points called ports. The influence a layout algorithm has on the placement of ports is determined by port constraints defined on the corresponding node. In this paper we present approaches for integrating port constraints into the layer-based approach to graph drawing pioneered by Sugiyama et al. We show how our layout algorithm, called KLay Layered, progresses from relaxed to more restricted port constraint levels as it executes, and how established algorithms for crossing minimization and edge routing can be extended to support port constraints. Compared to the previous layout algorithms supporting ports, our algorithm produces fewer edge crossings and bends and yields pleasing results. We also explain and evaluate how layout algorithms can be kept simple by using the concept of intermediate processors to structure them in a modular way. A case study integrating our layout algorithm into UC Berkeleys Ptolemy tool illustrates how KLay Layered can be integrated into Java-based applications.

Improved layout for data flow diagrams with port constraints

The automatic generation of graphical views for data flow models and the efficient development of such models require layout algorithms that are able to handle their specific requirements. Examples include constraints on the placement of ports as well as the proper handling of nested models. We present an algorithm for laying out data flow diagrams that improves earlier approaches by reducing the number of edge crossings and bend points. We validate the quality of our algorithm with a range of models drawn from Ptolemy, a popular modeling tool for the design of embedded systems.

On Comments in Visual Languages

Visual languages based on node-link diagrams can be used to develop software and usually offer the possibility to write explanatory comments. Which node a comment refers to is usually not made explicit, but is implicitly clear to readers through placement and content. While automatic layout algorithms can make working with diagrams more productive, they tend to destroy such implicit clues because they are not aware of them and thus do not preserve the spatial relationship between diagram elements. Implicit clues thus need to be inferred and made explicit to be taken into account by layout algorithms. In this paper, we improve upon a previous paper on the subject [9], introducing further heuristics that aim to describe relations between comments and nodes. These heuristics mainly help to reduce the number of attachments of comments that should not be attached to anything. We also derive propositions on how developers of visual languages should integrate comments.

Automatic layout in the face of unattached comments

Visual languages based on node-link diagrams are widely used for systems modeling. As in textual languages, comments can make diagrams easier to understand. In the absence of an explicit attachment between comments and the diagram elements they relate to, that relationship is usually given implicitly by the manual placement of comments near the related elements. While algorithms for the automatic layout of diagrams can make working with diagrams more effective, they usually fail to preserve implicit attachments by placing comments at arbitrary positions. In this paper, we propose a comment attachment algorithm that extracts implicit attachments and makes them accessible to layout algorithms. We implemented the algorithm in an application for browsing Ptolemy diagrams and achieved success rates, i. e. attachments as intended by the user, of up to 90 %.

Label management: Keeping complex diagrams usable

Most visual languages are not purely graphical but include textual labels to complete the picture. However, in some languages labels tend to become rather long and thereby enlarge diagrams considerably. Since todays state-of-the-art development tools usually display diagrams in full detail, users must often scroll through the diagram or zoom out until the diagram fits inside the available drawing area, but then ceases to be legible. In this paper, we address this problem by examining ways to dynamically shorten the text of labels to keep the size of a diagram manageable. We introduce a number of label shortening strategies, explain ways to integrate them into diagram generation processes based on automatic layout algorithms, and explain their relation to the established focus and context approach which aims at solving a similar problem. We evaluate our strategies based on the SCChart visual language and an open-source, Eclipse-based modeling environment.

Using One-Dimensional Compaction for Smaller Graph Drawings

We use the technique of one-dimensional compaction as part of two new methods tackling problems in the context of automatic diagram layout: First, a post-processing of the layer-based layout algorithm, also known as Sugiyama layout, and second a placement algorithm for connected components with external extensions. We apply our methods to dataflow diagrams from practical applications and find that the first method significantly reduces the width of left-to-right drawn diagrams. The second method allows to properly arrange disconnected graphs that have hierarchy-crossing edges.

Counting Crossings for Layered Hypergraphs

Orthogonally drawn hypergraphs have important applications, e. g. in actor-oriented data flow diagrams for modeling complex software systems. Graph drawing algorithms based on the approach by Sugiyama et al. place nodes into consecutive layers and try to minimize the number of edge crossings by finding suitable orderings of the nodes in each layer. With orthogonal hyperedges, however, the exact number of crossings is not determined until the edges are actually routed in a later phase of the algorithm, which makes it hard to evaluate the quality of a given node ordering beforehand. In this paper, we present and evaluate two cross counting algorithms that predict the number of crossings between orthogonally routed hyperedges much more accurately than the traditional straight-line method.

KIELER: Building on automatic layout for pragmatics-aware modeling

Automatic layout is a key enabler for pragmatics-aware modeling, which refers to model-driven engineering with designer productivity in mind. This showpiece introduces an infrastructure for the integration of graph layout libraries and their configuration with regard to graphical views of modeling applications.

Edge Label Placement in Layered Graph Drawing.

Many visual languages based on node-link diagrams use edge labels. We describe different strategies of placing edge labels in the context of the layered approach to graph drawing and investigate ways of encoding edge direction in labels. We also report on the results of experiments conducted to investigate the effectiveness of the strategies.

Incremental diagram layout for automated model migration

A range of successful modeling tools to develop complex systems use node-link-style diagrams as their underlying language. Over the years such languages can change, for instance as part of a tool update. When migrating existing models, changes in syntax directly affect the placement of elements in their diagrams. Increasing the size of certain nodes may for example result in node overlaps. In this paper we propose two methods based on graph drawing techniques to adjust the layout of existing diagrams after migration. Although we designed these techniques for diagram migration, they are applicable to other scenarios as well, such as users interactively adding or resizing nodes. We evaluate the techniques based on real world diagrams from the LabVIEW suite and discuss the scenarios each technique seems best suited for.