[Example] Hello World(Official)

    """Hello world for Sapien.

    Concepts:
        - Engine and scene
        - Renderer, viewer, lighting
        - Run a simulation loop

    Notes:
        - For one process, you can only create one engine and one renderer.
    """

    import sapien.core as sapien
    from sapien.utils import Viewer
    import numpy as np


    def main():
        engine = sapien.Engine()  # Create a physical simulation engine
        renderer = sapien.SapienRenderer()  # Create a renderer
        engine.set_renderer(renderer)  # Bind the renderer and the engine

        scene = engine.create_scene()  # Create an instance of simulation world (aka scene)
        scene.set_timestep(1 / 100.0)  # Set the simulation frequency

        # NOTE: How to build actors (rigid bodies) is elaborated in create_actors.py
        scene.add_ground(altitude=0)  # Add a ground
        actor_builder = scene.create_actor_builder()
        actor_builder.add_box_collision(half_size=[0.5, 0.5, 0.5])
        actor_builder.add_box_visual(half_size=[0.5, 0.5, 0.5], color=[1., 0., 0.])
        box = actor_builder.build(name='box')  # Add a box
        box.set_pose(sapien.Pose(p=[0, 0, 0.5]))


        # Add some lights so that you can observe the scene
        scene.set_ambient_light([0.5, 0.5, 0.5])
        scene.add_directional_light([0, 1, -1], [0.5, 0.5, 0.5])

        viewer = Viewer(renderer)  # Create a viewer (window)
        viewer.set_scene(scene)  # Bind the viewer and the scene

        # The coordinate frame in Sapien is: x(forward), y(left), z(upward)
        # The principle axis of the camera is the x-axis
        viewer.set_camera_xyz(x=-4, y=0, z=2)
        # The rotation of the free camera is represented as [roll(x), pitch(-y), yaw(-z)]
        # The camera now looks at the origin
        viewer.set_camera_rpy(r=0, p=-np.arctan2(2, 4), y=0)
        viewer.window.set_camera_parameters(near=0.05, far=100, fovy=1)

        while not viewer.closed:  # Press key q to quit
            scene.step()  # Simulate the world
            scene.update_render()  # Update the world to the renderer
            viewer.render()


    if __name__ == '__main__':
        main()