A utility for converting composite OBJ files to a MuJoCo model

[kevinzakka]

Introducing obj2mjcf, a CLI for processing composite Wavefront OBJ files into a MuJoCo-conducive format. It automatically:

• Splits an OBJ file into sub-meshes that are grouped by the materials referenced in the OBJ's MTL file
• Creates a collision mesh by performing a convex decomposition of the OBJ with V-HACD (if available on your path)
• Generates an MJCF XML file that is pre-filled with materials, meshes and geom elements referencing these OBJ files

The generated meshes can then be used as a drop-in replacement for the original OBJ file. The result is vastly enhanced visuals for your model:

Before	After

Why use this tool? As of June 2022, MuJoCo does not support composite OBJ files consisting of groups or objects (o or g OBJ tags) and only 1 material can be assigned per mesh. This means that you have to manually split your OBJ file into sub-meshes, a process that is tedious and error-prone. This tool is meant to automate this process.

Happy simulating,
Kevin

[yuvaltassa]

Thanks @kevinzakka, this is excellent!

In case someone asks the very legitimate question: "Will MuJoCo ever natively support loading composite OBJ files?" (as in discussion #255), the answer is no.

The reason for this is the following. MuJoCo loads meshes and textures separately, they have their own <asset> fields and are considered to be different types of assets. It is only in the <geom> that the mesh and texture are referenced and combined, but a geom cannot itself be loaded as an asset as it is a model element.

So given the current relationship of geoms and assets, Kevin's excellent solution is also the optimal one.