- Qt 6.4
- Qt Quick 3D Physics
- Qt Quick 3D Physics Examples and Tutorials
- Qt Quick 3D Physics - Mass Example
- Qt Quick 3D Physics | Commercial or GPLv3

# Qt Quick 3D Physics - Mass Example

Demonstrates different ways of setting mass and inertia of a body.

This example demonstrates three different ways of setting up the mass and inertia of a body. The scene consists of three bodies that consist of three spheres stacked vertically. These bodies all have the same mass but differing centers of mass and inertia tensors giving them different behavior when colliding.

#### Setup

We first add our DynamicsWorld:

     DynamicsWorld {
         running: true
         gravity: Qt.vector3d(0, -9.81, 0)
         typicalLength: 1
         typicalSpeed: 10
     }

We do the usual setup where we have an environment, camera and lights:

     environment: SceneEnvironment {
         clearColor: "lightblue"
         backgroundMode: SceneEnvironment.Color
     }

     PerspectiveCamera {
         id: camera
         position: Qt.vector3d(0, 2, 5)
         eulerRotation: Qt.vector3d(-10, 0, 0)
         clipFar: 100
         clipNear: 0.01
     }

     DirectionalLight {
         eulerRotation.x: -45
         eulerRotation.y: 45
         castsShadow: true
         brightness: 1
         shadowFactor: 100
     }

#### Physical objects

We have our regular static plane:

     StaticRigidBody {
         position: Qt.vector3d(0, 0, 0)
         eulerRotation: Qt.vector3d(-90, 0, 0)
         collisionShapes: PlaneShape {}
         Model {
             source: "#Rectangle"
             materials: DefaultMaterial {
                 diffuseColor: "green"
             }
             castsShadows: false
             receivesShadows: true
         }
     }

We define a custom QML class for our body which we call RolyPoly since they behave like so called roly-poly toys. The RolyPoly is a DynamicRigidBody with three spherical collision shapes:

     DynamicRigidBody {
         property string color: "blue"

         collisionShapes: [
             SphereShape {
                 id: sphere0
                 diameter:  1
             },
             SphereShape {
                 id: sphere1
                 diameter:  0.8
                 position: Qt.vector3d(0, 0.6, 0)
             },
             SphereShape {
                 id: sphere2
                 diameter:  0.6
                 position: Qt.vector3d(0, 1.1, 0)
             }
         ]

         Model {
             source: "#Sphere"
             position: sphere0.position
             scale: Qt.vector3d(1,1,1).times(sphere0.diameter*0.01)
             materials: PrincipledMaterial {
                 baseColor: color
             }
         }

         Model {
             source: "#Sphere"
             position: sphere1.position
             scale: Qt.vector3d(1,1,1).times(sphere1.diameter*0.01)
             materials: PrincipledMaterial {
                 baseColor: color
             }
         }

         Model {
             source: "#Sphere"
             position: sphere2.position
             scale: Qt.vector3d(1,1,1).times(sphere2.diameter*0.01)
             materials: PrincipledMaterial {
                 baseColor: color
             }
         }
     }

We then add three roly-polys to our scene:

     RolyPoly {
         position: Qt.vector3d(-2, 0.5, 0)
         color: "blue"

         mass: 0.9
         centerOfMassPosition: Qt.vector3d(0, -0.5, 0)
         inertiaTensor: Qt.vector3d(0.217011, 0.0735887, 0.217011)
         massMode: DynamicRigidBody.MassAndInertiaTensor
     }

     RolyPoly {
         position: Qt.vector3d(0, 0.5, 0)
         color: "purple"

         mass: 0.9
         centerOfMassPosition: Qt.vector3d(0, -0.5, 0)
         inertiaTensor: Qt.vector3d(0.05, 100, 100)
         massMode: DynamicRigidBody.MassAndInertiaTensor
     }

     RolyPoly {
         position: Qt.vector3d(2, 0.5, 0)
         color: "red"

         mass: 0.9
         massMode: DynamicRigidBody.Mass
     }

The purple and blue roly-poly has a custom center of mass and inertia tensor. Since bodies use uniform density by default and will calculate mass and inertia automatically we set massMode to DynamicRigidBody.MassAndInertiaTensor in our purple and blue bodies to use our provided mass and inertia tensors instead. The lower center of mass will make it so the bodies will always stand up after being pushed over. The inertia tensor of the purple body makes it so it will wobble easily in one direction but hardly in the other. The red body has an automatically calculated center of mass and will therefore keep lying down after being knocked over.

##### Shooting balls

To test out the behavior of the different bodies we add a Node for shooting balls:

     Node {
         id: shapeSpawner
         property var spheres: []

         function createBall(position, forward) {
             let diameter = 0.2;
             let speed = 20;
             let settings = {
                 position: position,
                 linearVelocity: forward.times(speed),
                 sphereDiameter: diameter
             };
             var component = Qt.createComponent("sphere.qml");
             let sphere = component.createObject(shapeSpawner, settings);

             var pair = {
                 "sphere" : sphere,
                 "date" : Date.now()
             };

             spheres.push(pair);

             if (sphere === null) {
                 console.log("Error creating object");
             }
         }

         function clean() {
             spheres = spheres.filter(sphere => {
                 let diff = Date.now() - sphere['date'];
                 if (diff > 5000) {
                     sphere['sphere'].destroy();
                     return false;
                 }
                 return true;
             });
         }

         Timer {
             interval: 200
             running: true
             repeat: true
             onTriggered: shapeSpawner.clean()
         }
     }

We then add a WasdController to be able to move the camera and aim and shoot balls at the bodies:

     WasdController {
         speed: 0.01
         controlledObject: camera
         Keys.onPressed: (event)=> {
             handleKeyPress(event);
             if (event.key === Qt.Key_Space) {
                 shapeSpawner.createBall(camera.position, camera.forward);
             }
         }
         Keys.onReleased: (event)=> { handleKeyRelease(event) }
     }

Files:

- mass/CMakeLists.txt
- mass/RolyPoly.qml
- mass/main.cpp
- mass/main.qml
- mass/mass.pro
- mass/qml.qrc
- mass/sphere.qml