Add nested models support in mujoco plugin by shameekganguly · Pull Request #1002 · gazebosim/gz-physics

shameekganguly · 2026-06-10T15:01:54Z

🎉 New feature

Part of #299

Summary

Mujoco plugin now supports nested models.

Implemented ConstructNestedModel in mujoco/src/SDFFeatures.cc
Handled nested model removal in mujoco/src/EntityManagementFeatures.cc
Added a new common test to check that link poses are correct for nested models after removing the models and adding them again (similar to the reset implementation in gz-sim Physics system). This is currently failing in bullet-featherstone. I will follow up with a fix.

Backport Policy

Can be backported with the rest of the PRs that add the Mujoco plugin.

Test it

gz sim -s -v 4 src/gz-sim/examples/worlds/nested_model.sdf --physics-engine gz-physics-mujoco-plugin

Checklist

Generated-by: Gemini 3.1 pro in Antigravity

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

Backports: If this is a backport, please use Rebase and Merge instead.

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

…ested models in mujoco plugin Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

azeey

I haven't finished reviewing yet, but I wanted to start a conversation about how self_collide tags are handled.

azeey · 2026-06-24T19:57:18Z

    // every pair
-    auto &objMap = modelInfo->links.objectToID;
-    for (auto it1 = objMap.begin(); it1 != objMap.end(); ++it1)
+    std::vector<const mjsBody *> allBodies;


This differs from the current behavior in DART. I did a quick test and came up with the following table that summarizes the behavior of the self_collide tag on a parent and nested models.

Parent flag nested model (child) flag note

true true Bodies in the parent collide against other bodies in the parent.
Bodies in the child collide against other bodies in the child.
Bodies in the parent collide against other bodies in the child and vice-versa.

true false Bodies in the parent collide against other bodies in the parent.
Bodies in child do not collide against each other.
Bodies in the parent collide against other bodies in the child and vice-versa.

false true Bodies in the parent do not collide against other bodies in the parent.
Bodies in the child collide against other bodies in the child.
Bodies in the parent collide against other bodies in the child and vice-versa

false false Bodies in the parent do not collide against other bodies in the parent.
Bodies in child do not collide against each other.
Bodies in the parent collide against other bodies in the child and vice-versa.

I believe what this code does is completely ignore the self_collide tag of nested models and treat all bodies as though they were in the parent model.

I've also created a simple SDF file to demonstrate the different behaviors with AI assistance:
https://gist.github.com/azeey/3f2392c0fc05a586d56869da23f28d40

All four cases are laid out side-by-side:

Case 1 (Parent: true, Child: true): Located at x = -4.5

Case 2 (Parent: true, Child: false): Located at x = -1.5

Case 3 (Parent: false, Child: true): Located at x = 1.5

Case 4 (Parent: false, Child: false): Located at x = 4.5

For each case, three tests run independently:

PP (Parent-Parent) at y = -1.5: A parent-level dropper (red sphere) falls onto a parent-level platform (red box).

CC (Child-Child) at y = 0.0: A nested child-level dropper (blue sphere) falls onto a nested child-level platform (blue box).

PC (Parent-Child) at y = 1.5: A nested child-level dropper (blue sphere) falls onto a parent-level platform (red box).

DART

Screen.Recording.2026-06-24.at.15.50.13.mov

MuJoCo

Screen.Recording.2026-06-24.at.15.50.59.mov

azeey · 2026-06-25T05:05:44Z

+      // Parent body is worldbody.
+      childPoseInParent = _rootSdfModel.RawPose() * childPoseInRoot;


This was discovered by Gemini, but I agree with it. The root model here might be a nested model itself. However, if we're the parent body is null, we need it's pose relative to the worldbody (absolute pose) rather than using RawPose which is the pose relative to its parent model.

* mujoco: Implement AttachFixedJointFeature and DetachJointFeature - Implemented `AttachFixedJoint` using MuJoCo equality weld constraints. - Optimized `AttachFixedJoint` and `DetachJoint` to reuse and toggle constraints (`active = 0/1`) to avoid expensive `mjModel` recompilations. - Implemented `CastToFixedJoint` to identify fixed joints. - Added comprehensive unit tests in `detachable_joint.cc`. Generated-by: Gemini 3.1 Pro --------- Signed-off-by: Addisu Z. Taddese <addisuzt@intrinsic.ai>

Set compiler.degree option to false to prevent MuJoCo from interpreting joint limits specified in radians as degrees. Also added a test case asserting the compiled joint limits of joint_limit_test. Generated-by: Gemini 3.5 Flash Signed-off-by: Addisu Z. Taddese <addisuzt@intrinsic.ai>

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

azeey

Disclaimer: I've used Gemini to help with the reviews.

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

azeey

Looks great! Thanks for iterating. Just one more small comment.

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly added 3 commits June 10, 2026 07:57

Add initial ConstructSdfNestedModel support to mujoco plugin

2b3be73

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Implement recursive model construction and collision exclusions for n…

10371c9

…ested models in mujoco plugin Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Support parent-scope joints between links in nested models

f4b19d2

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

azeey added this to Core development Jun 10, 2026

github-actions Bot added the 🎡 rotary label Jun 10, 2026

github-project-automation Bot moved this to Inbox in Core development Jun 10, 2026

shameekganguly added 9 commits June 10, 2026 11:10

Updated how joints with nested model child links are tracked

463315e

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Deduplicate nested model info in ModelInfo

cd7603c

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Handle nested model poses correctly

f8631c7

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Add a test for model pose after reset with nested models

bfd30c1

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Disable reset test case for bullet-featherstone

02de838

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Enable RemoveNestedModelCollisions test for mujoco

ae79fc3

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Fix lint

2b1e050

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Use existing nested model sdf

7b72d32

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Minor fixes

ee1d45e

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly force-pushed the shameek/mj_nested_models branch from 95e5e6d to cc76c26 Compare June 11, 2026 15:05

Reduce diff

bfcbfc9

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly force-pushed the shameek/mj_nested_models branch from cc76c26 to bfcbfc9 Compare June 11, 2026 15:09

shameekganguly added 2 commits June 11, 2026 08:15

Handle free floating link in nested model for ConstructNestedModel

579b0df

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Track parent model info in model info

040c800

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly marked this pull request as ready for review June 11, 2026 17:50

shameekganguly requested review from azeey and scpeters as code owners June 11, 2026 17:50

iche033 reviewed Jun 12, 2026

View reviewed changes

Comment thread mujoco/src/SDFFeatures.cc Outdated

Comment thread mujoco/src/SDFFeatures.cc Outdated

shameekganguly added 2 commits June 12, 2026 14:38

Optimizations

d5f82ca

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

Lint fixes

205878c

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

iche033 approved these changes Jun 15, 2026

View reviewed changes

github-project-automation Bot moved this from Inbox to In review in Core development Jun 15, 2026

azeey requested changes Jun 24, 2026

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azeey reviewed Jun 25, 2026

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azeey and others added 3 commits June 26, 2026 08:28

Address review comments

c71af2b

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly force-pushed the shameek/mj_nested_models branch from bade604 to c71af2b Compare June 26, 2026 15:28

Variable name fix

1d2bc93

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly requested a review from azeey June 26, 2026 17:20

Merge branch 'main' into shameek/mj_nested_models

217cce4

azeey reviewed Jul 1, 2026

View reviewed changes

shameekganguly mentioned this pull request Jul 6, 2026

[Mujoco plugin] Implement GetNestedModelFromModel feature #1021

Open

Address review comments + fix GetModelName to return local name

3ebaa6a

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly requested a review from azeey July 6, 2026 21:07

azeey approved these changes Jul 8, 2026

View reviewed changes

Comment thread mujoco/src/EntityManagementFeatures.cc Outdated

Review fixes sans bullet changes

f7b29e0

Signed-off-by: Shameek Ganguly <shameek@intrinsic.ai>

shameekganguly force-pushed the shameek/mj_nested_models branch from 6e73e36 to f7b29e0 Compare July 9, 2026 14:38

Merge branch 'main' into shameek/mj_nested_models

39d4ec4

azeey enabled auto-merge (squash) July 9, 2026 16:24

azeey merged commit 06f474d into main Jul 9, 2026
14 checks passed

azeey deleted the shameek/mj_nested_models branch July 9, 2026 16:45

github-project-automation Bot moved this from In review to Done in Core development Jul 9, 2026

Parent flag	nested model (child) flag	note
true	true	Bodies in the parent collide against other bodies in the parent. Bodies in the child collide against other bodies in the child. Bodies in the parent collide against other bodies in the child and vice-versa.
true	false	Bodies in the parent collide against other bodies in the parent. Bodies in child do not collide against each other. Bodies in the parent collide against other bodies in the child and vice-versa.
false	true	Bodies in the parent do not collide against other bodies in the parent. Bodies in the child collide against other bodies in the child. Bodies in the parent collide against other bodies in the child and vice-versa
false	false	Bodies in the parent do not collide against other bodies in the parent. Bodies in child do not collide against each other. Bodies in the parent collide against other bodies in the child and vice-versa.

		// Parent body is worldbody.
		childPoseInParent = _rootSdfModel.RawPose() * childPoseInRoot;

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shameekganguly commented Jun 10, 2026 • edited by azeey Loading Uh oh! There was an error while loading. Please reload this page.

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🎉 New feature

Summary

Backport Policy

Test it

Checklist

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azeey Jun 24, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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azeey Jun 25, 2026

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shameekganguly commented Jun 10, 2026 •

edited by azeey

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