The MuJoCo exporter works as Blender addon, tested with ver 3.4.1. You can download Blender from https://www.blender.org.

The install.sh command will deploy the installable version of the plugin to ./addons/mujoco_model_exporter. This folder should be zipped:

$ cd ./addons && zip -r mujoco_model_exporter.zip mujoco_model_exporter/*

Open Blender and select Edit/Preferences menu option.

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Clicking the Install button will open a file selection dialog that will allow you to select the .zip archive with the plugin.

The next window will follow, listing the installed plugin and allowing you to enable it.

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Check the checkbox next to its name to enable it.

The plugin was designed to allow the artists to keep using core Blender features when building models for Mujoco.

The exporter will copy the names of the bones, meshes and lights. Mujoco joints will be named after the bone that owns the respective constraint, with a postfix denoting the degree of freedom they enable.

Blender Armatures allow to model kinematic capabilities of a Blender model. They comprise a tree of bones, each of which can be further extended with IK Constraints.

Only the bones affected by an IK chain will receive appropriate degrees of freedom. This means that if you export a model without any IK chains, it will be exported as a static model.

IK constraints offer the easiest way to verify how a kinematic chain would behave in a physical environment. For that reason we chose to use them, along with IK bone limits, to model Mujoco joints.

• Select the bone at the end of a kinematic chain,

• Add an IK Bone Constraint to it. This webpage contains detailed instructions.

• Add a target bone to the scene and use it as the IK constraint's target.

• Adjust the Chain Length value to reflect the number of parent bones that should become a part of this IK chain.

• For each of the bones in the chain, visit the Bone/Inverse Kinematics tab and adjust the locks and bone limits.

NOTE: you can take advantage of a helpful limit visualization gizmo.

Armatures form the kinematic trees, so it makes sense to give their roots all 6 degrees of freedom.

This behavior can be changed by disabling the Armature freejoint export option.

Mujoco uses parametric geometry comprised of primitive shapes such as cubes, spheres and capsules. It does support triangle based mesh geometry however.

Blender on the other hand deals exclusively in meshes, and even though it contains a palette of such primitive shapes, these are not parametric.

The exporter therefore exports the geometry as meshes. All meshes referenced by the scene are exported into Mujoco's native .msh format.

When this plugin was created, the native .msh format was the only format to support texture mapping. Since MuJoCo 2.1.2, .obj files are supported. The .msh format is expected to be deprecated and removed soon. Until this plugin is updated to output .obj files we recommend all user to convert their .msh files to .obj files using this utility.

Mujoco implements a forward rendering model with Phong lightning and support for reflective surfaces.

Mujoco material definition is fixed and limited to:

• base color (diffuse color)
• specularity coefficient
• smoothness coefficient
• reflectiveness coefficient

These parameters translate well to Blender's default material definition, which should be used to model the materials exported to Mujoco.

Due to the diversity and open-ended nature of Cycle's material nodes, the choice was made not to support materials defined using them.

Mujoco renderer supports a single material per mesh, in contrast to Blender's multi material model. In order to support meshes with multiple materials, the exporter divides them into submeshes, each of which uses a single material.

Since Blender materials are applied to faces, and a single face may only have a single material assigned, there is no risk that the subdivided mesh will exhibit any overlapping artifacts.

The exported submeshes will have different geometry and volume than the original. This will affect the mass and inertia of the geom that uses the exported mesh.

Please note that Mujoco derives these quantities not from the mesh itself, but rather from the convex hull calculated for it. The sum of hulls of subdivided meshes are not guaranteed to be equal to the hull of the original mesh.

This phenomenon is illustrated on the image below.

Blender	Mujoco renderer	Mujoco convex hulls

A different material was assigned to 2 out of 6 rectangular faces of the right cube, causing that mesh to be split into two submeshes.

While they render well in Mujoco, the Moiré effect visible on the convex hull comes from two overlapping hulls, each with a different volume.

Double sided materials cause the meshes (or faces) that use them to be exported with the faces duplicated with the reverse face winding order. This is because Mujoco renderer does not support any other face culling modes than back face culling.

CAUTION: Because this operation affects the exported geometry, it may indirectly affect the physical properties of a geom that references that mesh, such as its mass and inertia. Please use this feature with caution.

CAUTION: If you are using the scaling transform, the exporter will modify your scene!

The exporter will by default reset the scaling transform on all bones and meshes to ensure affine reference frame transformations.

This operation can be undo'ed after the exporter is completed, but the exporter doesn't undo it automatically!

NOTE: This feature will be added in the next version of the exporter.

Mujoco renderer supports a single texture channel. It's therefore advised to use a single UV map for meshes that are to be exported to Mujoco.

Mujoco's fixed rendering pipeline is quite fast, and handles both variants well. Therefore we leave it up to the artist to decide which it prefers using.

Please keep in mind that the exporter doesn't export the texture assets. Those should be copied into the folder with the exported model manually.