Skip to content

Defining a Ground Surface

Abstract

The rocket needs some ground to collide with, we will be defining a custom class which will contain the key elements needed to configure the ground.

Ground preview
A horizontal plane acting as the ground. The ground has a checkerboard pattern and a shadow of the rocket tube (yet to be defined) cast from the light added in the previous step.

Class Definition

The Ground class is (thankfully) pretty straightforward. We define three things:

  1. Grid Texture: A checkerboard texture used on the ground plane. It is technically not needed, but helpful to give things a sense of scale.
  2. Material: To apply a texture to a geometry (like our ground plane) we apply it via a material. This gives options for other photorealistic rendering textures, but we will stick with the simple built-in one for now.
  3. Geometry: A GeomPlane (a special built-in geometry type) is used to make a contact surface for the rocket. It can be positioned in whatever orientation is needed.
    • For a more complex model, you could even use a heightmap to simulate an uneven surface!
Python
 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
class Ground(mojo.UserData):
    """Definition of the ground plane."""

    @classmethod
    def new(cls, mojo_model: mojo.MojoModel) -> Self:
        assert mojo_model.mjcf.worldbody  # For type hinting

        # Configure ground grid and plane
        mojo_model.mjcf.assets.append(
            mojo.Asset(
                textures=[
                    grid_tex := mojo.TextureBuiltIn(
                        name=mojo.TextureName("grid_tex"),
                        type=mojo.TextureType.D2,
                        builtin=mojo.TextureBuiltInType.CHECKER,
                        width=1024,
                        height=1024,
                        rgb1=mojo.utils.Color.SLATE_400.rgb,
                        rgb2=mojo.utils.Color.SLATE_600.rgb,
                    )
                ],
                materials=[
                    grid_mat := mojo.Material(
                        name=mojo.MaterialName("grid_mat"),
                        texture=grid_tex.name,
                        texrepeat=np.array((1, 1)),
                    )
                ],
            )
        )
        mojo_model.mjcf.worldbody.geoms.append(
            mojo.GeomPlane(
                name=mojo.GeomName("floor"),
                size=np.array([0, 0, 0.1]),
                pose=mojo.PoseQuat(pos=np.array((0.5 * US.m, 0.5 * US.m, 0))),
                material=grid_mat.name,
            ),
        )

        return cls()

Success

We have defined a ground plane, its contact behavior, and applied it to our mojo_model! We are now ready to begin building the rocket and landing gear assembly.