Sugiyama layout - chunhualiao/public-docs GitHub Wiki

1. Current "Tree Layout" in This Project

  • The project implements a "tree" or "hierarchical" layout, as seen in examples/tkinter_demo.py and related files.
  • This layout:
    • Assigns nodes to levels based on tree depth (root at the top, children below).
    • Calculates subtree widths to space siblings and avoid overlap.
    • Handles multiple roots (forests) and includes some cycle detection to avoid infinite recursion.
    • Is designed for tree-like graphs (i.e., acyclic, hierarchical structures).
    • Does not minimize edge crossings for general graphs.
    • Does not insert dummy nodes for long edges or handle cycles in the way Sugiyama-style does.
    • For non-tree graphs (with cycles, cross-links, or arbitrary DAGs), the layout may be suboptimal: edges may cross excessively, and the visual hierarchy may be unclear.

2. Sugiyama-Style Layered Layout (as in ZGRViewer/Graphviz "dot")

  • Designed for general directed graphs (not just trees).
  • Key features:
    • Layer assignment: Nodes are assigned to discrete layers (ranks) to minimize edge length and preserve directionality.
    • Dummy nodes: Inserted for edges that span multiple layers, ensuring all edges connect adjacent layers.
    • Edge crossing minimization: Uses heuristics (e.g., barycenter, median) to reorder nodes within layers to reduce crossings.
    • Coordinate assignment: Assigns x/y positions to minimize width/height and crossings.
    • Cycle handling: Breaks cycles (temporarily) to produce a DAG for layout.
  • Advantages:
    • Produces clear, readable layouts for arbitrary directed graphs, not just trees.
    • Minimizes edge crossings, which is critical for large or complex graphs.
    • Is the standard for DOT graph visualization (used by Graphviz's "dot" and ZGRViewer).

3. Comparison

Feature Current Tree Layout Sugiyama-Style Layered Layout
Handles general graphs No (trees/forests only) Yes (DAGs, general digraphs)
Edge crossing minimization No Yes
Dummy node insertion No Yes
Cycle handling Basic (cycle detection) Yes (cycle breaking)
Layer assignment By tree depth By rank assignment (minimizing edge length)
Visual clarity for DAGs Poor High
Used in ZGRViewer/dot No Yes

4. Is It Worth Implementing Sugiyama-Style?

If your use case involves:

  • Only small, simple trees: The current layout is sufficient.
  • Large trees, forests, or simple hierarchies: The current layout is still fine, but edge crossings may become an issue if there are cross-links.
  • General directed graphs, DAGs, or graphs with cycles: The Sugiyama-style layout is vastly superior. It will produce much more readable and professional visualizations, especially for complex or real-world DOT files.

In summary:

  • The current tree layout is a good, simple solution for pure trees.
  • For anything more complex (DAGs, general digraphs, or DOT files with cross-links), implementing a Sugiyama-style layered layout is highly recommended and will bring your visualizations in line with industry standards (Graphviz, ZGRViewer).
  • Implementing Sugiyama-style is a non-trivial task, but there are many references and open-source implementations (e.g., Graphviz's "dot" source, networkx's graphviz_layout, or academic papers).

Recommendation

  • If your users expect ZGRViewer/Graphviz-like results, or you want to support general DOT files, it is worth implementing the Sugiyama-style layered layout.
  • If you only ever visualize simple trees, the current layout is sufficient.

Optional: Visual Comparison (Mermaid)

flowchart TD
    subgraph Tree_Layout
        A1[Root]
        A2[Child 1]
        A3[Child 2]
        A4[Grandchild]
        A1 --> A2
        A1 --> A3
        A2 --> A4
    end
    subgraph Sugiyama_Layout
        B1[Node 1]
        B2[Node 2]
        B3[Node 3]
        B4[Node 4]
        B1 --> B2
        B1 --> B3
        B2 --> B4
        B3 --> B4
    end
  • In the Sugiyama layout, B4 is placed to minimize crossings, and all edges flow in one direction, even with cross-links.

If you want to proceed with implementing Sugiyama-style, I can help you plan the steps or suggest libraries and algorithms.