seating-a-cable-crossing

Seat one crossing, then look, and hand on an honest number.

Why this is its own skill

A weave that streams its legs and hopes never finds out which crossing failed. The whole difference between a full solve and a third of one on the weave bench was one crossing that did not take and was never looked at again. This skill closes that loop for a single crossing: stream the planner's place leg, wait for the rod to come to rest, then segment the rod, fit its centreline and count the densified curve inside the station's seat box.

"Did not take" is a fraction, not a touch. The task scores containment as the share of the rod inside a box a few centimetres wide beside the station, on the side the route requires (>= 0.047 on the weave bench). Measured on the failures, a losing station is almost never on the wrong side: it is on the right side with too little of it there.

What it deliberately does not do

It measures and does not correct. The obvious correction — push the hand further onto the required side when the box is short — was built and measured and made the seat worse (0.016 to 0.000 after a 14 mm nudge): the cable is not short of the box, it is crossing it at an angle, and driving the contact point further in rotates it further. A correction wants a different move, arriving along the seat rather than across it, and that belongs in the carry planner.

Routing

The script always returns route: done; the verdict is in the outputs. seated is fraction >= seat_fraction; fraction is 0.0 when the seat could not be measured at all (no camera frame, no depth, no rod mask, a fit too short to be a curve) and detail says so. A graph that wants to branch on the verdict reads seated, not the exit. blocked is the exit a subgraph declares as on_error for a tool that raises during the place or the look.

Which station

The plan's station field names the station the planner actually worked, and that — not the loop's pass index — selects the seat box. The planner does not work stations in pass order (far-side crossings first, near-side seats last, each from the anchored end outwards), so indexing the perceived list by the pass number measured a different station's box from the one just worked on every pass. index is the fallback only when the plan carries no station.

Inputs and parameters

Input	Shape
plan	JSON: {"stages": [{"stage": "place", "trajectory": Trajectory, ...}], "station": int, "side": ±1} from the crossing planner
scene	JSON: {"stations": [[x, y], ...], "sides": [±1, ...]} from the fixture survey
index	the pass number; used only when the plan names no station
arm_id	the work arm that streams the place leg

Parameters with the weave bench's numbers as defaults: seat_fraction (0.047), seat_inner / seat_outer (0.003 / 0.045 m out from the station on the required side), seat_half_y (0.035 m along the station line), camera ("cable"), rod_query ("thin white cable"), curve_nodes (42). Read a different bench's off its own task document rather than reusing them. grip_open_m / grip_close_m are accepted for uniform binding and unused.

What a CPU test can and cannot exercise

The fraction arithmetic (densification, the side sign, the box bounds) and the plan-station indexing run against canned observations and a canned centreline fit. What a CPU test cannot exercise is the perception behind sam3.segment_text and curve.fit_centerline on a real frame, and whether a streamed place leg actually lays cable where the plan said.

Required end states

End state	Meaning
done	Streamed and measured; read seated / fraction / detail.
blocked	A tool raised; the subgraph's on_error.