A user's approval-workflow flowchart came out with the return arrow drawn
straight through two unrelated steps, and a second report showed arrows
leaving a box and bending straight back across it. Measured over 250
generated flowcharts (2722 edges): 347 arrows crossed an unrelated box and
215 waypoints landed inside a shape. Four defects, each measured in
isolation:
1. The invisible layer containers draw_graph emits were handed to the
router as frames, so every clean return path was rejected for
"trespassing" on a border that is not drawn, and the fallback cut
through two boxes. Excluding invisible containers: 347 -> 218 crossing
arrows, no diagram made worse.
2. The router chose the horizontal-vs-vertical axis BEFORE searching, so
when the only clean corridor ran along the other axis it was never
looked at. A complete two-bend candidate generator that tries both
trunk axes, all four sides at each end, and the port fractions:
218 -> 27. (An independent ablation measured the axis pre-choice alone
at a 40% per-edge failure rate.)
3. Nothing stopped a route's first leg from turning back across its own
source shape - the obstacle test exempts an edge's own endpoints, and
must, since the line has to touch them. A terminal-leg rule refuses
such routes outright: 215 -> 4 hooks.
4. A two-bend search cannot express the staircase needed when a box sits
directly between two vertically aligned nodes (21 of the last 27
crossings). Added the orthogonal visibility graph + A* from Wybrow,
Marriott & Stuckey, "Orthogonal Connector Routing" (GD 2009) - the
libavoid algorithm - as the backstop when the candidate search finds
nothing. The interesting-points grid is provably sufficient: any valid
route shrinks onto it without getting longer or gaining bends. The A*
state is (point, incoming direction) with libavoid's bend cost of 10,
and the admissible bends-remaining heuristic, so it returns a cheapest
route, not merely a route. Implemented from the paper, not ported.
After all four: 0 crossing arrows and 0 hooks over the same 250 diagrams,
page area unchanged (494k px^2 mean), 260ms for the whole corpus, mean
0.82 bends per edge. The shape ladder still runs first, so routes that
were already clean are byte-identical.
Also post-nudge validation now checks the whole path (the nudge pass only
reverts the single segment it moved, judged in isolation) and restores the
search's route if nudging made it dirty.
