5 Graph Description Language (GDL) - Absint

6.4 Hierarchical Layout
6.4.1 Rank Assignment
After folding, all the visible nodes are determined. If all the visible nodes have been specified by
the user using valid coordinates, the graph is drawn immediately. However, if the coordinates of
at least one node are missing, an appropriate layout has to be calculated. The first pass places the
nodes in discrete ranks. All nodes of the same rank appear at the same vertical position.
There are many possibilities for assigning rank. The normal method is to calculate a spanning
tree by determining the strongly connected components of the graph. All edges should be oriented
top-down. A heuristic tries to find a minimum set of edges that cannot be oriented top-down.
A faster method is to calculate the spanning tree of a graph by depth first search (DFS). However,
the order in which the nodes are visited has a substantial influence on the layout. The initial order
of the nodes is the order given by the graph specification. aiSee offers various versions of such
methods:
• dfs:
Calculates the spanning tree by one single DFS traversal. This is the fastest method, but the
quality of the result might be poor for some graphs.
• maxdepth:
Calculates the spanning tree by DFS using the initial order and the reverted initial order,
followed by choosing the deepest spanning tree. This results in more levels, i.e. the graph is
larger in the y direction.
• mindepth:
Takes the flatter spanning tree of both DFS’s. This results in fewer levels, or more nodes at
the same levels, meaning the graph is larger in the x direction.
• maxdepthslow, mindepthslow:
Whereas the above algorithms are fast heuristics for increasing or decreasing the depth of
the layout, maxdepthslow and mindepthslow actually calculate a good order so as to obtain
a maximum or minimum spanning tree. However, they pose one disadvantage: they are
rather slow. Warning: A minimum spanning tree does not necessarily mean that the depth
of the layout is minimal. However, good heuristics involves obtaining a flat layout (see the
examples on p. 53).
• maxdegree, mindegree, maxindegree, minindegree
maxoutdegree, minoutdegree:
These algorithms combine DFS with node sorting. The sorting criteria are the number of
incoming edges, the number of outgoing edges, and the number of edges all at the same
node. Node sorting may have various effects and can sometimes be used as a fast alternative
to maxdepthslow or mindepthslow.
• minbackward:
Instead of calculating strongly connected components, aiSee can also perform topological
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