Description

Fix triangle-mesh-vs-convex collisions silently dropping all contacts under non-uniform (and even large uniform) mesh scale. The mesh BVH is built over unscaled mesh.points, but the AABB query in mesh_vs_convex_midphase was performed in scaled mesh-local space. Any convex shape whose unscaled-space AABB lay outside the BVH bounds received 0 triangles and 0 contacts. The query AABB and per-axis contact gap are now converted from scaled to unscaled (BVH) space before the BVH overlap test.

Also adds two small helper funcs in collision_core.py:

• aabb_to_unscaled — convert an AABB from a component-wise scaled frame to the unscaled frame, swapping per-axis min/max under negative scale.
• transform_normal_with_scale — correct normal transform under non-uniform component-wise scale (R · (n / scale), normalized).

Checklist

• New or existing tests cover these changes
• The documentation is up to date with these changes
• CHANGELOG.md has been updated (if user-facing change)

Test plan

New regression class TestMeshNonUniformScaling in newton/tests/test_narrow_phase.py drops a sphere onto a unit-quad mesh with various scales (uniform, large uniform, pancake (10,10,1), thin (1,1,0.1), extreme (50,0.5,1)) and asserts contacts are produced with sane unit-length, roughly +Z normals; a far-separated case asserts 0 contacts (no false positives).

uv run --extra dev -m newton.tests -k test_narrow_phase.TestMeshNonUniformScaling

Bug fix

Triangle-mesh-vs-convex collisions silently produce zero contacts whenever the mesh shape uses a non-uniform scale (e.g. (10, 10, 1)), and even for sufficiently large uniform scales. Mesh-averaging of the scale caused the AABB query against the BVH (which lives in unscaled mesh-local space) to miss all triangles.

Steps to reproduce:

1. Build a triangle mesh and add it as a shape with a non-uniform scale=(sx, sy, sz) (e.g. (10, 10, 1)).
2. Place a convex shape (sphere/box/capsule/...) so it overlaps the scaled mesh in world space.
3. Run the narrow phase — observe contact_count == 0.

Minimal reproduction:

import numpy as np
import newton

verts = np.array(
    [[-0.5, -0.5, 0.0], [0.5, -0.5, 0.0], [0.5, 0.5, 0.0], [-0.5, 0.5, 0.0]],
    dtype=np.float32,
)
inds = np.array([0, 1, 2, 0, 2, 3], dtype=np.int32)
mesh = newton.Mesh(verts, inds)

# Non-uniform scale: 10x10 quad in world space, sphere dropped just above it.
# Without this fix: 0 contacts. With this fix: contacts as expected.

Summary by CodeRabbit

• Bug Fixes

  • Fixed triangle-mesh-vs-convex collisions to correctly handle non-uniform and large uniform mesh scaling, preventing silent loss of contact detection.

• Tests

  • Added comprehensive test coverage for mesh-vs-convex collision behavior under various scaling scenarios, including non-uniform and extreme anisotropic scaling configurations.