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code/snow_rl/snow_env.py

@@ -148,7 +148,12 @@ class SnowJoystick(joystick.Joystick):
         if slope != 0.0:
             # Rotate gravity about +y: the robot walking along +x is then walking uphill.
             g = float(np.linalg.norm(self._mj_model.opt.gravity))
-            tilted = np.array([g * np.sin(slope), 0.0, -g * np.cos(slope)])
+            # NEGATIVE x component: gravity pulls the robot BACK along -x while the
+            # walking command drives +x, so positive slope_rad means walking UPHILL.
+            # The sign was inverted first time round, which made gravity assist the command
+            # -- passive drift measured +1.02 m downhill at 17 deg, and "slope performance"
+            # improved with steepness because the robot was being pushed along.
+            tilted = np.array([-g * np.sin(slope), 0.0, -g * np.cos(slope)])
             self._mj_model.opt.gravity[:] = tilted
             self._mjx_model = self._mjx_model.replace(opt=self._mjx_model.opt.replace(
                 gravity=jnp.asarray(tilted)))