# Description

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This PR ports in the functions for procedurally generating different
terrains. The terrains can be generated as heightfields (2.5 D elevation
map) or meshes (through `trimesh`).

There are two separate classes:
* `TerrainGenerator`: This class only procedurally generates terrains
based on the passed sub-terrain configuration. It creates the trimesh
object and contains the origins of each terrain in the mesh.
Additionally, it performs caching of the sub-terrain meshes based on
their configuration. By default, caching is disabled since most
developers will change not only the config, but also the function.
* `TerrainImporter`: This class mainly deals with importing terrains
from different possible sources and adding them to the simulator. In
addition to importing terrains, it also stores them into a dictionary
called `warp_meshes` that later can be used for ray-casting. The types
of terrain sources are as follows:
* `import_ground_plane`: creates a grid plane which is default in
isaacsim/orbit.
    * `import_mesh`: takes a trimesh object
    * `import_usd`: takes a USD file and adds reference to it

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## Type of change

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- New feature (non-breaking change which adds functionality)

## Screenshots


![terrains](https://github.com/isaac-orbit/orbit/assets/12863862/05f889d0-b98e-4063-a810-89d9116c2dfe)

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## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file

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