fix(assembly): fix planar half-space correction causing drag flip with cylindrical joints #331

forbes · 2026-02-26T13:46:53Z

forbes commented

2026-02-26 13:46:53 +00:00

Problem

When dragging a body connected by a Cylindrical joint + Distance=0 PlanePlane constraint, every drag step is rejected as 'flipped orientation' (>91 degrees). The solver converges fine but the C++ validator rejects the result, making the part appear frozen during drag.

Root Cause

In mods/solver/kindred_solver/preference.py, the _planar_half_space() function builds a half-space tracker for PlanarConstraint. When distance=0 (point already on the plane), it uses dot(z_i, z_j) as its indicator and provides a correction function that reflects the body through the plane.

When the body rotates about the cylindrical axis, its planar face normal rotates with it. After ~90 degrees of legitimate rotation, the dot product crosses zero, triggering the correction which teleports the body to the opposite side of the plane — a massive orientation change that the 91-degree threshold rightfully rejects.

Fix

Return a tracking-only HalfSpace (no correction_fn) when the point is already on the plane. This matches the pattern used by Cylindrical, Revolute, and Concentric half-space trackers. The cross-product residuals in PlanarConstraint already enforce normal alignment via the solver, so the correction is redundant and harmful during drag.

Testing

All 286 solver tests pass.

See: kindred/solver#38

## Problem When dragging a body connected by a **Cylindrical joint** + **Distance=0 PlanePlane constraint**, every drag step is rejected as 'flipped orientation' (>91 degrees). The solver converges fine but the C++ validator rejects the result, making the part appear frozen during drag. ## Root Cause In `mods/solver/kindred_solver/preference.py`, the `_planar_half_space()` function builds a half-space tracker for PlanarConstraint. When distance=0 (point already on the plane), it uses `dot(z_i, z_j)` as its indicator **and provides a correction function** that reflects the body through the plane. When the body rotates about the cylindrical axis, its planar face normal rotates with it. After ~90 degrees of legitimate rotation, the dot product crosses zero, triggering the correction which teleports the body to the opposite side of the plane — a massive orientation change that the 91-degree threshold rightfully rejects. ## Fix Return a **tracking-only HalfSpace** (no correction_fn) when the point is already on the plane. This matches the pattern used by Cylindrical, Revolute, and Concentric half-space trackers. The cross-product residuals in PlanarConstraint already enforce normal alignment via the solver, so the correction is redundant and harmful during drag. ## Testing All 286 solver tests pass. See: kindred/solver#38

forbes added 1 commit 2026-02-26 13:46:54 +00:00

fix(assembly): update solver submodule — fix planar half-space drag flip

Build and Test / build (pull_request) Successful in 29m44s

Details

d5af9804ec

Updates mods/solver to include fix for the planar half-space correction
that caused 'flipped orientation' rejections when dragging a body
connected by a Cylindrical joint + distance=0 Planar constraint.

The solver's PlanarConstraint half-space tracker was reflecting the body
through the plane when the face normal dot product crossed zero during
legitimate rotation about the cylindrical axis. Now returns a tracking-
only HalfSpace (no correction) for on-plane constraints, matching the
pattern used by Cylindrical/Revolute/Concentric trackers.

See: kindred/solver#38

forbes force-pushed fix/planar-halfspace-drag-flip from d5af9804ec to 559a240799

2026-02-26 15:04:35 +00:00

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forbes commented

2026-02-26 16:33:16 +00:00

Not a valid solution to constraint failure problem.

forbes closed this pull request

2026-02-26 16:33:16 +00:00

Build and Test / build (pull_request) Has been cancelled

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Reference: kindred/create#331