Donovan’s Vision Blog

Adding gravity

BlockStack is now available on the App Store! Try the game now.

Following on from adding a drag gesture, we can make it respond to gravity using a PhysicsBodyComponent.

Starting with a simple reflective cube this time, adjusting from the sphere in the drag example. I've also adjusted the scale and position to make it easier to see. In the video above, it's being shown in the "museum" environment as it has a good amount of floor space. This demo won't account for furniture.

RealityView { content in
    // Cube entity
    cube = ModelEntity(
        mesh: .generateBox(size: 0.1),
            materials: [SimpleMaterial(color: .white, isMetallic: true)])
    // Scale and position
    cube.scale = [3, 3, 3]
    cube.position.y = 1
    cube.position.z = -4

    // Components
    cube.components.set(GroundingShadowComponent(castsShadow: true))
    cube.components.set(InputTargetComponent())

    // Collisions
    cube.generateCollisionShapes(recursive: true)


    content.add(cube)
}
.gesture(
DragGesture()
    .targetedToEntity(cube)
    .onChanged { value in
        cube.position = value.convert(value.location3D, from: .local, to: orb.parent!)
    }
)

Adding physics #

The cube can be dragged around and will stay position when released. We can add some gravity physics to make it fall:

let physicsMaterial = PhysicsMaterialResource.generate(
    staticFriction: 0.8,
    dynamicFriction: 0.5,
    restitution: 0.01 // How bouncy
)
cube.components[PhysicsBodyComponent.self] = .init(
    massProperties: .default,
    material: physicsMaterial,
    mode: .dynamic
)

In this we're creating a PhysicsMaterialResource which allows us to specify friction and restition (bounciness). A low value here means it'll behave like a solid heavy block of metal. A higher value and it'll bounce around more.

We apply this PhysicsMaterialResource as a material within the PhysicsBodyComponent component, and set mode to dynamic so that it will move.

Creating a floor #

Without a floor, the cube just falls away. We can add a floor entity:

let floor = ModelEntity(
    mesh: .generatePlane(width: 50, depth: 50),
    materials: [OcclusionMaterial()]
)
floor.generateCollisionShapes(recursive: false)
floor.components[PhysicsBodyComponent.self] = .init(
    massProperties: .default,
    mode: .static
)
content.add(floor)

Here we generate a ModelEntity made of a plane and using an OcclusionMaterial. This is an invisible material which hides any objects rendered behind it.

With the physics enabled, the objects will fall and land on a "floor".

Finished code #

RealityView { content in
    // Floor
    let floor = ModelEntity(
        mesh: .generatePlane(width: 50, depth: 50),
        materials: [OcclusionMaterial()]
    )
    floor.generateCollisionShapes(recursive: false)
    floor.components[PhysicsBodyComponent.self] = .init(
        massProperties: .default,
        mode: .static
    )
    content.add(floor)

    // Cube
    cube = ModelEntity(
        mesh: .generateBox(size: 0.1),
            materials: [SimpleMaterial(color: .white, isMetallic: true)])
    // Scale and position
    cube.scale = [3, 3, 3]
    cube.position.y = 1
    cube.position.z = -4

    // Components
    cube.components.set(GroundingShadowComponent(castsShadow: true))
    cube.components.set(InputTargetComponent())

    // Collisions
    cube.generateCollisionShapes(recursive: true)

    // Physics
    let physicsMaterial = PhysicsMaterialResource.generate(
        staticFriction: 0.8,
        dynamicFriction: 0.5,
        restitution: 0.01 // How bouncy
    )
    cube.components[PhysicsBodyComponent.self] = .init(
        massProperties: .default,
        material: physicsMaterial,
        mode: .dynamic
    )

    content.add(cube)
}
.gesture(
DragGesture()
    .targetedToEntity(cube)
    .onChanged { value in
        cube.position = value.convert(value.location3D, from: .local, to: orb.parent!)
    }
)