adds known issue on stepping

docs/source/refs/issues.rst

@@ -1,6 +1,35 @@
 Known issues
 ============
 
+Stale values after resetting the environment
+--------------------------------------------
+
+When resetting the environment, some of the data fields of assets and sensors are not updated.
+These include the poses of links in a kinematic chain, the camera images, the contact sensor readings,
+and the lidar point clouds. This is a known issue and which has to do with the way the PhysX and
+rendering engines work in Omniverse.
+
+Many physics engines do a simulation step as a two-level call: ``forward()`` and ``simulate()``,
+where the kinematic and dynamic states are updated, respectively. Unfortunately, PhysX has only a
+single ``step()`` call where the two operations are combined. Due to computations through GPU
+kernels, it is not so straightforward for them to split these operations. Thus, at the moment,
+it is not possible to set the root and/or joint states and do a forward call to update the
+kinematic states of links. This affects both initialization as well as episodic resets.
+
+Similarly for RTX rendering related sensors, the sensor data is not updated immediately after
+setting the state of the sensor. The rendering engine update is bundled with the simulator's
+``step()`` call which only gets called when the simulation is stepped forward. This means that
+the sensor data is not updated immediately after a reset.
+
+While the above is erroneous, there is currently no direct workaround for it. From our experience in
+using IsaacGym, the reset values are crucial. However, it does not affect the agent learning
+critically enough.
+
+We have made a feature request to the respective Omniverse teams to have complete control
+over stepping different parts of the simulation app. However, at this point, there is no set
+timeline for this feature request.
+
+
 Blank initial frames from the camera
 ------------------------------------