[MISC] Allow following entity or mounting camera building scene. (#1548)

Author: duburcqa

genesis/utils/misc.py

@@ -142,7 +142,7 @@ def assert_built(method):
     @functools.wraps(method)
     def wrapper(self, *args, **kwargs):
         if not self.is_built:
-            gs.raise_exception("Scene is not built yet.")
+            gs.raise_exception(f"{type(self).__name__} is not built yet.")
         return method(self, *args, **kwargs)
 
     return wrapper

genesis/vis/rasterizer_context.py

@@ -74,6 +74,8 @@ def build(self, scene):
         self.scene = scene
         self.sim = scene.sim
         self.visualizer = scene.visualizer
+
+        # Update visuals at this point avoids nasty visual artifacts during Scene build
         self.visualizer.update_visual_states()
 
         if self.rendered_envs_idx is None:

genesis/vis/visualizer.py

@@ -215,10 +215,11 @@ def update_visual_states(self):
             return
 
         for camera in self._cameras:
-            if camera._attached_link is not None:
-                camera.move_to_attach()
-            elif camera._followed_entity is not None:
-                camera.update_following()
+            if camera.is_built:
+                if camera._attached_link is not None:
+                    camera.move_to_attach()
+                elif camera._followed_entity is not None:
+                    camera.update_following()
 
         if self._scene.rigid_solver.is_active():
             self._scene.rigid_solver.update_geoms_render_T()