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Current architecture : Svek
This current architecture (which codename is Svek) uses Graphviz/Dot to compute nodes position. The drawing itself is fully done in Java.
The main advantage is that, since PlantUML does the drawing, the software is now less dependent of Graphviz/Dot.
The code is also simpler, because the two steps (node computation then graphical rendering) are clearly separated.
With this architecture, Graphviz/Dot generates a simplified SVG output (whatever is the actual output format need by the user), and those simplified SVG data are parsed by PlantUML. The drawing is then done by PlantUML.