Current limitations of URDF -> MJCF converter

[saikishor]

I'm creating this issue to document and collect the opinions of others who are having trouble generating the final MJCF from the URDF. The idea is to use it to improve the current converter

[SergiMuac]

Hi!
I have a URDF model that currently uses mesh-based collision geometries, which work correctly but introduce significant computational overhead during simulation. A simplified MJCF version was created using capsule collision geometries instead, resulting in substantially improved performance.

An attempt was made to define capsule collision geometry directly within the URDF, expecting the URDF-to-MJCF converter to preserve or translate this information into the resulting MJCF file. However, the converter fails with the following error:

Exception: Invalid geometric tag: capsule

The issue appears to arise because capsule is a valid geometry type in MuJoCo but not part of the URDF specification. Consequently, the converter interprets it as invalid URDF syntax and crashes.

Suggested improvement

It would be highly beneficial if the converter supported one of the following:

• A mechanism to override or replace collision meshes during conversion (e.g., mapping mesh collisions to simpler primitives such as capsules), or
• Detection and handling of MuJoCo-specific geometry tags (such as capsule) embedded in URDF files, allowing them to be translated appropriately into the generated MJCF.

Such functionality would enable performance optimizations without requiring manual post-processing of the MJCF output.