aiSee: Reducing Layout Time

Reducing layout time

aiSee was designed to explore large graphs. However, the layout of large graphs may require considerable time. Thus, there are many possibilities to speed up the layout algorithm: Parts of the graph can be folded, iterations can be limited, and time limits can be specified.

The first step in visualizing a large graph is avoiding computing the layout of parts of the graph that are currently not of interest. These parts should be specified as initially folded in the specification (see GDL in a Nutshell). Folding makes the visible part of the graph smaller, thus enabling the layout to be calculated faster and the quality of the layout to be improved. It is useful to first try one of the fast algorithms (dfs, minbackward, tree), then the medium fast methods (normal, mindepth, maxdepth, ...) before resorting to the slower methods (mindepthslow, maxdepthslow).

In order to further reduce layout time, some layout phases should be omitted or the maximum number of iterations of some layout phases limited. However, this usually decreases the quality of the layout. First, the crossing reduction phase 2 should be skipped as it usually takes the most time (command line option -nocopt2, GDL attribute crossing_phase2). Crossing reduction phase 2 is indicated by B in the message window (see message characters). Next, the iterations for the crossing reduction can be limited (-cmax option, cmax attribute) or another crossing weight selected (command line options -bary, -barymedian, etc., GDL attribute crossing_weight). Normally, it is not necessary to switch off local crossing optimization, because this step is quite fast and effective.

If the graph is very unbalanced, the pendulum method may require considerable time (indicated by m in the message window). In this case, the number of iterations for the crossing reduction should be limited (-pmax option, pmax attribute). If the straight-line fine-tuning phase takes too long (indicated by S in the message window), its maximum iteration number should be limited (-smax option, smax attribute).

Other parameters usually needn't be changed, because the corresponding phases are quite fast. Bending reduction in particular improves layout quality greatly and is so fast that the -bending option is hardly needed.

Fast mode (-fast option) drastically reduces all iteration limits and may result in poor layout quality.

The drawing of splines is slow, consequently it should be avoided on large graphs.

It is also possible to set a time limit (-timelimit option). If the time limit is exceeded, the fastest possible mode for the current and the following iteration phases is selected. Selecting a time limit does not mean that the layout has actually finished when the time has elapsed: Layout may be faster if the graph is small and/or its structure is simple. It may be much slower, because even the fastest possible methods may take some time. The time limit is real time, thus the result of the layout may depend on the load of the computer. Thus, given a time limit, two identical trials needn't result in identical layouts.

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