Fixes divide by zero issue when setting gravity to zero (#399)

# Description

This PR fixes divide by zero issue when setting gravity to (0, 0, 0).

It was causing unintended effects - position and linear velocity
remained nan even when modifying root state via
`RigidObject.write_root_state_to_sim()`.

Fixes #398

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## 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 run all the tests with `./orbit.sh --test` and they pass
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.10.21"
+version = "0.10.22"
 
 # Description
 title = "ORBIT framework for Robot Learning"

source/extensions/omni.isaac.orbit/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.10.22 (2024-02-14)
+~~~~~~~~~~~~~~~~~~~~
+
+Fixed
+^^^^^
+
+* Fixed "divide by zero" bug in :class:`~omni.isaac.orbit.sim.SimulationContext` when setting gravity vector.
+  Now, it is correctly disabled when the gravity vector is set to zero.
+
+
 0.10.21 (2024-02-12)
 ~~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/simulation_cfg.py

@@ -190,7 +190,10 @@ class SimulationCfg:
     """The number of physics simulation steps per rendering step. Default is 1."""
 
     gravity: tuple[float, float, float] = (0.0, 0.0, -9.81)
-    """The gravity vector (in m/s^2). Default is (0.0, 0.0, -9.81)."""
+    """The gravity vector (in m/s^2). Default is (0.0, 0.0, -9.81).
+
+    If set to (0.0, 0.0, 0.0), gravity is disabled.
+    """
 
     enable_scene_query_support: bool = False
     """Enable/disable scene query support for collision shapes. Default is False.

source/extensions/omni.isaac.orbit/omni/isaac/orbit/sim/simulation_context.py

@@ -492,7 +492,13 @@ class SimulationContext(_SimulationContext):
         #  need to convert the gravity vector to a direction and magnitude pair explicitly.
         gravity = np.asarray(self.cfg.gravity)
         gravity_magnitude = np.linalg.norm(gravity)
-        gravity_direction = gravity / gravity_magnitude
+
+        # Avoid division by zero
+        if gravity_magnitude != 0.0:
+            gravity_direction = gravity / gravity_magnitude
+        else:
+            gravity_direction = gravity
+
         physics_scene.CreateGravityDirectionAttr(Gf.Vec3f(*gravity_direction))
         physics_scene.CreateGravityMagnitudeAttr(gravity_magnitude)
 

source/extensions/omni.isaac.orbit/test/sim/test_simulation_context.py

@@ -125,6 +125,16 @@ class TestSimulationContext(unittest.TestCase):
         sim.clear_instance()
         self.assertEqual(ctypes.c_long.from_address(id(sim)).value, sim_ref_count - 1)
 
+    def test_zero_gravity(self):
+        """Test that gravity can be properly disabled."""
+        cfg = SimulationCfg(gravity=(0.0, 0.0, 0.0))
+
+        sim = SimulationContext(cfg)
+
+        gravity_dir, gravity_mag = sim.get_physics_context().get_gravity()
+        gravity = np.array(gravity_dir) * gravity_mag
+        np.testing.assert_almost_equal(gravity, cfg.gravity)
+
 
 if __name__ == "__main__":
     try: