getBoundElbowAxes

For each bound terminal, the axis of the normal of the bound shape's nearest edge: "x" on a left or right edge, "y" on a top or bottom one. An elbow arrow routes its first and last leg along these, so it leaves and enters a bound shape square-on instead of diagonally. A free terminal is absent from the result and the router falls back to the dominant axis.

The bound shape's bounds are measured in arrow space, so a rotated shape is compared against the box it occupies from the arrow's point of view — which is the frame the route itself is built in.