Question about SDF lib plugin and making SDF in general

[benthebear93]

Intro

Hi!

I am a graduate student at SDU, I use MuJoCo for my research on Contact manipulation.
For few days, i have been trying to get a good collision mesh for bettetr contact.

I found few ways to do it.

1. Write your own SDFs and make it as a plugin
2. Use obj2mjcf (or CoACD, same thing) to decompose Non convex obj files.
3. Use "mujoco.sdf.sdflib" plugin and set geom type="sdf"

First one may be the best option to have good performance but required too many work.
Second one works pretty fine but if i want to have finner collision meshes, i needed to decompose more which makes too many obj files for one object.

Third one works good without having bunch of objs but i couldn't really find the documents about SDF lib plugin.

My setup

MuJoCo 3.2.0
Python
Linux 22.04

My question

Is third method just same plugin as first one but default?

What are the differences between the methods mentioned above?
Thank you.

Minimal model and/or code that explain my question

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[quagla]

Hi, the collision algorithm for methods 1 and 3 will be the same. The difference is that 1 requires writing a new plugin, while 3 means using the already existing sdflib plugin (like in this example https://github.com/google-deepmind/mujoco/blob/main/model/plugin/sdf/cow.xml) - the underlying representation of the SDF is a voxel grid computed from the mesh using this library https://github.com/UPC-ViRVIG/SdfLib

Using 3 is likely not as fast as using 1 but it's obviously more general because it's really hard to come up with an analytic SDF for complicated objects, so computing it automatically using a voxel grid is a big advantage.

[yuvaltassa]

That said, @quagla we should probably document the sdflib business somewhere?

[benthebear93]

Thanks for the information :)