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Add lock_inertia flag to ModelBuilder.add_link, add_body#1422

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eric-heiden merged 1 commit into
newton-physics:mainfrom
eric-heiden:body-inertia-locking
Jan 21, 2026
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Add lock_inertia flag to ModelBuilder.add_link, add_body#1422
eric-heiden merged 1 commit into
newton-physics:mainfrom
eric-heiden:body-inertia-locking

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@eric-heiden eric-heiden commented Jan 20, 2026 •
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This flag (defaults to False) will prevent shapes to contribute mass, COM, inertia to a body.
Fixes #1330.

Summary by CodeRabbit

  • New Features

    • Added lock_inertia parameter to body and link creation methods, enabling users to prevent automatic mass and inertia updates when shapes are subsequently added to locked bodies.

  • Tests

    • Added tests verifying inertia locking behavior for shape addition and fixed joint collapse operations.

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@eric-heiden
Add lock_inertia flag to ModelBuilder.add_link, add_body
076a156 
Signed-off-by: Eric Heiden <eric-heiden@outlook.com>
@eric-heiden eric-heiden requested a review from vastsoun January 20, 2026 20:24
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📝 Walkthrough

Walkthrough

The PR introduces a per-body lock_inertia flag in ModelBuilder that prevents subsequent shape additions from modifying a body's mass, center of mass, or inertia. The flag is added as a parameter to add_body() and add_link(), propagated through joint collapses and builder merges, and respected during finalization operations.

Changes

Cohort / File(s) Summary
Core Builder Implementation
newton/_src/sim/builder.py		 Added body_lock_inertia: list field to track per-body locking state. Extended add_body() and add_link() with new lock_inertia parameter. Updated inertia computation paths (finalize, _update_body_mass, shape inertia) to skip updates for locked bodies. Propagated lock flag through collapse_fixed_joints() and dfs() merge operations.
Unit Tests
newton/tests/test_model.py		 Added test_lock_inertia_on_shape_addition() to verify locked bodies preserve mass/com/inertia when shapes are added. Added test_collapse_fixed_joints_with_locked_inertia() to verify lock state is preserved when collapsing fixed joints.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

  • Add inertia verification, correction #440 #455: Modifies ModelBuilder's inertia handling paths (finalize, mass/inertia updates) alongside the main PR's per-body lock flag implementation; both affect overlapping code paths for inertia verification and updates during builder finalization.
🚥 Pre-merge checks | ✅ 4 | ❌ 1
❌ Failed checks (1 warning)
Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title accurately describes the main change: adding a lock_inertia flag parameter to two public methods.
Linked Issues check ✅ Passed The PR fully addresses issue #1330 by implementing the proposed lock_inertia flag on add_body and add_link that prevents shape additions from modifying mass/inertia/COM.
Out of Scope Changes check ✅ Passed All changes are within scope: lock_inertia parameter addition, internal propagation through merge/collapse logic, and comprehensive test coverage for the feature.

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codecov Bot commented Jan 20, 2026

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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@eric-heiden eric-heiden marked this pull request as ready for review January 20, 2026 23:47
@eric-heiden eric-heiden self-assigned this Jan 21, 2026
vastsoun
vastsoun approved these changes Jan 21, 2026
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@vastsoun vastsoun left a comment

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LGTM, seems to do the trick.

I think, however, that this solution is not ideal. We could consider in a future refactoring/reworking of the ModelBuilder, to handle this in a more systematic manner. For example, we could define some logic similar to that of USD, where automatic re-calculations happen only based on the data provided to the builder etc, and re-calculate if something is missing from the body specs, as well as to add a helper function that users can call to perform the recalculations.

@eric-heiden eric-heiden added this pull request to the merge queue Jan 21, 2026
Merged via the queue into newton-physics:main with commit 622a8c3 Jan 21, 2026
22 checks passed
jnskkmhr pushed a commit to jnskkmhr/newton that referenced this pull request Jan 22, 2026
@eric-heiden @jnskkmhr
Add lock_inertia flag to ModelBuilder.add_link, add_body (newton-ph…
82f335e 
…ysics#1422)

Signed-off-by: Eric Heiden <eric-heiden@outlook.com>
eric-heiden added a commit to eric-heiden/newton that referenced this pull request Jan 28, 2026
@eric-heiden
Add lock_inertia flag to ModelBuilder.add_link, add_body (newton-ph…
82f0074 
…ysics#1422)

Signed-off-by: Eric Heiden <eric-heiden@outlook.com>
@eric-heiden eric-heiden mentioned this pull request Feb 4, 2026
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@coderabbitai coderabbitai Bot mentioned this pull request Feb 6, 2026
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@coderabbitai coderabbitai Bot mentioned this pull request Feb 25, 2026
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mmacklin pushed a commit to mmacklin/newton that referenced this pull request Apr 7, 2026
@eric-heiden
Add lock_inertia flag to ModelBuilder.add_link, add_body (newton-ph…
f57040c 
…ysics#1422)

Signed-off-by: Eric Heiden <eric-heiden@outlook.com>
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Prevent overwriting body mass from shapes in ModelBuilder

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