Various small utilities I've created while working with MuJoCo.

Converts an existing MuJoCo skin A.skn and a Blender B.obj into a new MuJoCo skin using B's vertices, interpolated on A's vertices using nearest-neighbor interpolation.

One thing that you might want to do with skins is to create them from a Blender mesh, saved as an .obj. But that is tough, since there's no GUI for weighting vertices and it's obviously a cumbersome task to do on your own. One solution, if you already have a MuJoCo skin, is to interpolate to it. That's what my script is doing.

If you have an A.skn and a B.obj, you should first make sure that the vertices are roughly in the same position, so that B mesh can be meaningfully projected onto A by nearest-vertex interpolation. If A.skn was generated from an .stl/.msh/.obj file, you can do this by just loading that file into Blender and aligning your B mesh with it, and saving the result.

Download reskin.py, make sure that B.mtl (the .mtl associated with B.obj) is in the same directory as B.obj, and run script in Python like this:

>>> python reskin.py path1/to/A.skn path2/to/B.obj path3/to/B.skn 

It will save the reskinned mesh B.skn into path3/to.

Starting from 2.0, MuJoCo has a new type of rendering mesh called skin and associated with the .skn extension. It is used for example in dm_control's Dog model. Calling them meshes can be slightly misleading; they don't participate in the physics, and are exclusively used for rendering. They follow a custom binary format described here.

In mathy terms, a skin consists of a mesh (collection of vertices $w = (x, y, z) \in {\bf R}^3$ and triangle faces $t = (w, w', w'')$ ), texcoords $w \mapsto (u_w, v_w) \in [0, 1]^2$ and bones $b$, which provide the connection to MuJoCo's physics; each bone is associated with a MuJoCo body, and holds a list of vertices $W(b)$ whose position it affects, and associated weights $a_{b, w}$ for $w \in W(b)$. So if each bone $b$ moves by $\Delta b$, then a vertex $w$ may be expected to move by

$$ \sum_{w \in W(b)} a_{b, w} \cdot \Delta b. $$

The texcoords are of course for wrapping a bitmap on a mesh. What my script does is taking a new set of vertices, $z$, and first calculating the nearest neighbor $w(z)$, and then setting $a_{b, z} = a_{b, w(z)}$ for all $z$, and $(u_z, v_z) = (u_{w(z)}, u_{v(z)})$.