5. Simulator Building Blocks

• 5. Simulator Building Blocks
  • 5.1. SimRepository
  • 5.2. SimFactory
  • 5.3. On the topic of Simulation Objects

If you have browsed some of the various demos, you have seen that all the demo files begin with a similar code section:

from alr_sim.sims.SimFactory import SimRepository

### Create Robot and Scene. REQUIRED!
sim_factory = SimRepository.get_factory('pybullet')
pb_robot = sim_factory.create_robot()
scene = sim_factory.create_scene(pb_robot, object_list=object_list)
scene.start()

The advantage of this approach is that one can quickly swap out the simulators by changing the identifier in the .get_factory() line. The simulator internals remain hidden.

In this guide we want to deconstruct and explain this code section, and how it creates a Robot and Scene object.

5.1. SimRepository

The SimRepository object is a global registry for all the different physics engines (simulators) which the ALR SimulationFramework currently supports. you can use SimRepository.list_all_sims() to query which simulators are supported AND have been installed on your system. The list of supported simulators include to date (07/2021):

• Main Simulators
  • pybullet - the pybullet simulator
  • mujoco - the MuJoCo simulator
• "Derived" Simulators
  • mujoco_mocap - a MuJoCo simulation, but with the Mocap Control option for the robot.
  • interactive - a MuJoCo simulation, but you can directly control the robot in real-time using an XBOX Controller. Should be considered experimental.

You can use these string identifiers to retrieve the corresponding SimFactory object from the SimRepository.

5.2. SimFactory

The SimFactory objects provide an abstraction layer for the creation of the Simulator specific building blocks:

• a simulator specific Scene implementation
• a simulator specific Robot implementation
• a simulator specific Camera implementation

To get an overview of their public functions, it might be worth to take a look at the base classes in the core package.

5.3. On the topic of Simulation Objects

It is not possible for us to provide a similar abstraction for creating arbitrary objects for a scene. Each simulator has vastly different approaches on how object properties such as visual shapes, collision shapes or even multi-body systems are defined.

In the PrimitiveObjects module, we provide you with primitive shapes such as Boxes, Spheres and Cylinders. If you look at the code, you can see that they implement three interfaces:

class PrimitiveObject(SimObject, PybulletLoadable, MujocoLoadable):
  ...

The SimObject interface makes it accessible to read the object states such as position and orientation from the simulations.

The PybulletLoadable interface defines the function, which the PybulletScene will call to load it into the scene.

The MujocoLoadable interface defines the function, which the MujocoScene will call to load it into the scene.

When you look into the code for the PrimitiveObject you will see, why it is not possible for us to have a truly universal way to load objects into our scenes without defining a new kind of meta layer for each simulator.

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