FA-87969 / Inverse kinematics solvers / Member archive

FABRIK reachability is measured from the current tip · case 04

Reachable targets are stretched toward as unreachable, and far targets near the tip enter the iteration.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Input [points, target, tol, iters]: planar joint positions from base to tip. Link lengths come from the initial points. If the base-to-target distance exceeds the total length, stretch every link toward the target and return ["unreachable", points]. Otherwise run FABRIK passes (tip to target and backward, base re-anchored, forward) while the tip is farther than tol and fewer than iters passes ran; return [status, passes, points] with status "reached" when the final tip error <= tol else "max-iterations"; coordinates rounded to 4 places.

Why this case matters

Inverse kinematics code turns task-space goals into joint commands; a wrong branch, sign, limit or update order sends a real arm to the wrong pose.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
near full stretch reachable	`["unreachable", [[0.0, 0.0], [0.9976, 0.0688], [2.9929, 0.2064]]]`	`["reached", 15, [[0.0, 0.0], [0.9105, 0.4134], [2.8991, 0.2001]]]`	Failed

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