Rigid bodies: the source

since 0.2.0 Physics and particles
Open the live demo · Read the guide · View on GitHub
Regions the guide quotes: world (line 41), body (line 63), snapshot (line 104), live (line 114), shove (line 133), restore (line 142), check (line 168).
1/// Crates dropped one after another onto a floor: they land, stack, settle and
2/// fall asleep, and a snapshot taken before any of it winds the pile back to
3/// where it began. The physics runs live, a step to a frame.
4///
5/// Quoted by `rigid_bodies.md` and shown whole in the Source tab.
6library;

8import 'dart:math' as math;

10import 'package:flutter3d/flutter3d.dart';
11import 'package:flutter3d_physics/flutter3d_physics.dart';
12import 'package:flutter3d_showcase/src/demo/demo.dart';
13import 'package:vector_math/vector_math.dart';

15final class RigidBodiesDemo extends ShowcaseDemo {
late final CollisionWorld _world;
late final Dynamics _dynamics;
final List<RigidBody> _crates = <RigidBody>[];
final List<double> _halves = <double>[];
final List<MeshNode> _meshes = <MeshNode>[];
final List<Map<String, Object?>> _start = <Map<String, Object?>>[];

bool _shoveAsked = false;
double _atRest = 0.0;
double _age = 0.0;

static const int _count = 8;
static const double _step = 1 / 60;

@override
void configureView(DemoContext context) {
  context.orbit
    ..distance = 8.0
    ..pitch = 0.35
    ..yaw = 0.6;
  context.orbit.target.setValues(0.0, 1.0, 0.0);
}

@override
Scene build(DemoContext context) {
  _world = CollisionWorld();
  _world.addBox(Vector3(0.0, -0.5, 0.0), Vector3(6.0, 1.0, 6.0));
  _dynamics = Dynamics(world: _world);

  final Scene scene = Scene()
    ..ambientColor = Vector3(0.5, 0.55, 0.65)
    ..ambientIntensity = 0.25
    ..add(
      MeshNode(
        DeviceMesh.upload(
          context.device,
          CuboidShape(size: Vector3(6.0, 1.0, 6.0)).build(),
        ),
        Material(name: 'floor', baseColor: Vector4(0.42, 0.46, 0.43, 1.0)),
        name: 'floor',
      )..setPosition(0.0, -0.5, 0.0),
    )
    ..add(
      LightNode(name: 'sun', intensity: 2.5)
        ..setLocalForward(Vector3(-0.3, -0.6, -0.4)),
    );

  // Eight crates, each higher up and a little off to one side of the last,
  // so they arrive one at a time and land on one another as often as on
  // the floor.
  for (var i = 0; i < _count; i++) {
    final double half = 0.3 + 0.08 * (i % 3);
    final RigidBody crate = _dynamics.add(
      RigidBody(
        world: _world,
        shape: CollisionBox(Vector3(half, half, half)),
        position: Vector3(
          0.5 * math.sin(i * 2.4),
          1.2 + i * 1.1,
          0.5 * math.cos(i * 1.9),
        ),
        mass: 2.0,
        friction: 0.7,
      ),
    );
    _crates.add(crate);
    _halves.add(half);
    final MeshNode mesh = MeshNode(
      DeviceMesh.upload(
        context.device,
        CuboidShape(size: Vector3.all(half * 2)).build(),
      ),
      Material(
        name: 'crate $i',
        baseColor: Vector4(
          0.55 + 0.4 * math.sin(i * 0.9),
          0.5 + 0.3 * math.sin(i * 1.7 + 1.0),
          0.35 + 0.3 * math.sin(i * 2.3 + 2.0),
          1.0,
        ),
        roughness: 0.7,
      ),
      name: 'crate $i',
    )..setPositionFrom(crate.position);
    _meshes.add(mesh);
    scene.add(mesh);
  }

  // Taken before a single step: everything a body needs to be put back
  // exactly here — position, velocity, whether it is asleep.
  for (final RigidBody crate in _crates) {
    _start.add(crate.save());
  }
  return scene;
}

@override
void update(DemoContext context, double dt) {
  _age += dt;
  // A step of a sixtieth of a second, however long the frame took: the
  // same drop lands the same way on every machine.
  _dynamics.step(_step);

  if (_shoveAsked) {
    _shoveAsked = false;
    _shove();
  }

  _atRest = _crates.every((RigidBody c) => c.isAsleep) ? _atRest + dt : 0.0;
  // Watched for a moment at rest, then wound back and dropped again.
  if (_atRest > 1.5 || _age > 12.0) _rewind();

  for (var i = 0; i < _count; i++) {
    _meshes[i].setPositionFrom(_crates[i].position);
  }
}

void _shove() {
  for (var i = 0; i < _count; i++) {
    _crates[i].applyImpulse(
      Vector3(math.sin(i * 2.1) * 3.0, 4.0, math.cos(i * 1.7) * 3.0),
    );
  }
  _atRest = 0.0;
}

void _rewind() {
  for (var i = 0; i < _count; i++) {
    _crates[i].restore(_start[i]);
  }
  _atRest = 0.0;
  _age = 0.0;
}

@override
List<DemoControl> controls(DemoContext context) => <DemoControl>[
  ToggleControl(
    'Shove them',
    value: () => _shoveAsked,
    onChanged: (bool v) => _shoveAsked = v,
  ),
  ToggleControl(
    'Rewind to the start',
    value: () => false,
    onChanged: (bool v) {
      if (v) _rewind();
    },
  ),
];

@override
void verify(Scene scene, FrameResult frame) {
  // Let the pile come to rest. Every crate has to end up on the floor or
  // on another crate: none fallen through, none left hanging in the air.
  for (var i = 0; i < 600; i++) {
    _dynamics.step(_step);
  }
  for (var i = 0; i < _count; i++) {
    final double y = _crates[i].position.y;
    if (y < _halves[i] - 0.05) {
      throw StateError('crate $i fell through the floor: y = $y');
    }
    if (y > 8.0) {
      throw StateError('crate $i is still in the air: y = $y');
    }
  }
  if (!_crates.every((RigidBody c) => c.isAsleep)) {
    throw StateError('the pile should have come to rest and gone to sleep');
  }

  // Winding back has to undo all of it, exactly.
  _rewind();
  for (var i = 0; i < _count; i++) {
    final Vector3 back = _crates[i].position;
    final Vector3 saved = Vector3(
      0.5 * math.sin(i * 2.4),
      1.2 + i * 1.1,
      0.5 * math.cos(i * 1.9),
    );
    if ((back - saved).length > 1e-6) {
      throw StateError('restore should have put crate $i back where it was');
    }
  }
  if (frame.drawCalls < 1) {
    throw StateError('the crates did not reach the frame');
  }
}
203}