examples/game/lib/game_demo_world.dart - mirrors/flutter - Git at Google

 part of game;

 const double _steeringThreshold = 0.0;
 const double _steeringMax = 150.0;

 // Random generator
 Math.Random _rand = new Math.Random();

 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;

 const double _shipRadius = 30.0;
 const double _lrgAsteroidRadius = 40.0;
 const double _medAsteroidRadius = 20.0;
 const double _smlAsteroidRadius = 10.0;
 const double _maxAsteroidSpeed = 1.0;

 const int _lifeTimeLaser = 50;

 const int _numStarsInStarField = 150;

 class GameDemoWorld extends NodeWithSize {

   // Images
   Image _imgNebula;

   SpriteSheet _spriteSheet;

   // Inputs
   double _joystickX = 0.0;
   double _joystickY = 0.0;
   bool _fire;

   Node _gameLayer;

   Ship _ship;
   List<Asteroid> _asteroids = [];
   List<Laser> _lasers = [];
   StarField _starField;
   Nebula _nebula;

   GameDemoWorld(ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {

     // Fetch images
     _imgNebula = images["res/nebula.png"];

     _gameLayer = new Node();
     this.addChild(_gameLayer);

     // Add some asteroids to the game world
     for (int i = 0; i < 5; i++) {
       addAsteroid(AsteroidSize.large);
     }
     for (int i = 0; i < 5; i++) {
       addAsteroid(AsteroidSize.medium);
     }

     // Add ship
     addShip();

     // Add starfield
     _starField = new StarField(_spriteSheet["star.png"], _numStarsInStarField);
     _starField.zPosition = -2.0;
     addChild(_starField);

     // Add nebula
     addNebula();

     userInteractionEnabled = true;
     handleMultiplePointers = true;
   }

   // Methods for adding game objects

   void addAsteroid(AsteroidSize size, [Point pos]) {
     Asteroid asteroid = new Asteroid(_spriteSheet["asteroid_big_1.png"], size);
     asteroid.zPosition = 1.0;
     if (pos != null) asteroid.position = pos;
     _gameLayer.addChild(asteroid);
     _asteroids.add(asteroid);
   }

   void addShip() {
     Ship ship = new Ship(_spriteSheet["ship.png"]);
     ship.zPosition = 10.0;
     _gameLayer.addChild(ship);
     _ship = ship;
   }

   void addLaser() {
     Laser laser = new Laser(_spriteSheet["laser.png"], _ship);
     laser.zPosition = 8.0;
     _lasers.add(laser);
     _gameLayer.addChild(laser);
   }

   void addNebula() {
     _nebula = new Nebula.withImage(_imgNebula);
     _gameLayer.addChild(_nebula);
   }

   void update(double dt) {
     // Move asteroids
     for (Asteroid asteroid in _asteroids) {
       asteroid.position = pointAdd(asteroid.position, asteroid._movementVector);
     }

     // Move lasers and remove expired lasers
     for (int i = _lasers.length - 1; i >= 0; i--) {
       Laser laser = _lasers[i];
       laser.move();
       if (laser._frameCount > _lifeTimeLaser) {
         laser.removeFromParent();
         _lasers.removeAt(i);
       }
     }

     // Apply thrust to ship
     if (_joystickX != 0.0 || _joystickY != 0.0) {
       _ship.thrust(_joystickX, _joystickY);
     }

     // Move ship
     _ship.move();

     // Check collisions between asteroids and lasers
     for (int i = _lasers.length -1; i >= 0; i--) {
       // Iterate over all the lasers
       Laser laser = _lasers[i];

       for (int j = _asteroids.length - 1; j >= 0; j--) {
         // Iterate over all the asteroids
         Asteroid asteroid = _asteroids[j];

         // Check for collision
         if (pointQuickDist(laser.position, asteroid.position) < laser.radius + asteroid.radius) {
           // Remove laser
           laser.removeFromParent();
           _lasers.removeAt(i);

           // Add asteroids
           if (asteroid._asteroidSize == AsteroidSize.large) {
             for (int a = 0; a < 3; a++) addAsteroid(AsteroidSize.medium, asteroid.position);
           }
           else if (asteroid._asteroidSize == AsteroidSize.medium) {
             for (int a = 0; a < 5; a++) addAsteroid(AsteroidSize.small, asteroid.position);
           }

           // Remove asteroid
           asteroid.removeFromParent();
           _asteroids.removeAt(j);
           break;
         }
       }
     }

     // Move objects to center camera and warp objects around the edges
     centerCamera();
     warpObjects();
   }

   void centerCamera() {
     const cameraDampening = 0.1;
     Point delta = new Point(_gameSizeWidth/2 - _ship.position.x, _gameSizeHeight/2 - _ship.position.y);
     delta = pointMult(delta, cameraDampening);

     for (Node child in _gameLayer.children) {
       child.position = pointAdd(child.position, delta);
     }

     // Update starfield
     _starField.move(delta.x, delta.y);
   }

   void warpObjects() {
     for (Node child in _gameLayer.children) {
       if (child.position.x < 0) child.position = pointAdd(child.position, new Point(_gameSizeWidth, 0.0));
       if (child.position.x >= _gameSizeWidth) child.position = pointAdd(child.position, new Point(-_gameSizeWidth, 0.0));
       if (child.position.y < 0) child.position = pointAdd(child.position, new Point(0.0, _gameSizeHeight));
       if (child.position.y >= _gameSizeHeight) child.position = pointAdd(child.position, new Point(0.0, -_gameSizeHeight));
     }
   }

   // Handling controls

   void controlSteering(double x, double y) {
     _joystickX = x;
     _joystickY = y;
   }

   void controlFire() {
     addLaser();
   }

   // Handle pointer events

   int _firstPointer = -1;
   int _secondPointer = -1;
   Point _firstPointerDownPos;

   bool handleEvent(SpriteBoxEvent event) {
     Point pointerPos = convertPointToNodeSpace(event.boxPosition);
     int pointer = event.pointer;

     switch (event.type) {
       case 'pointerdown':
         if (_firstPointer == -1) {
           // Assign the first pointer
           _firstPointer = pointer;
           _firstPointerDownPos = pointerPos;
         }
         else if (_secondPointer == -1) {
           // Assign second pointer
           _secondPointer = pointer;
           controlFire();
         }
         else {
           // There is a pointer used for steering, let's fire instead
           controlFire();
         }
         break;
       case 'pointermove':
         if (pointer == _firstPointer) {
           // Handle turning control
           double joystickX = 0.0;
           double deltaX = pointerPos.x - _firstPointerDownPos.x;
           if (deltaX > _steeringThreshold || deltaX < -_steeringThreshold) {
             joystickX = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
             if (joystickX > 1.0) joystickX = 1.0;
             if (joystickX < -1.0) joystickX = -1.0;
           }

           double joystickY = 0.0;
           double deltaY = pointerPos.y - _firstPointerDownPos.y;
           if (deltaY > _steeringThreshold || deltaY < -_steeringThreshold) {
             joystickY = (deltaY - _steeringThreshold)/(_steeringMax - _steeringThreshold);
             if (joystickY > 1.0) joystickY = 1.0;
             if (joystickY < -1.0) joystickY = -1.0;
           }

           controlSteering(joystickX, joystickY);
         }
         break;
       case 'pointerup':
       case 'pointercancel':
         if (pointer == _firstPointer) {
           // Un-assign the first pointer
           _firstPointer = -1;
           _firstPointerDownPos = null;
           controlSteering(0.0, 0.0);
         }
         else if (pointer == _secondPointer) {
           _secondPointer = -1;
         }
         break;
       default:
         break;
     }
     return true;
   }
 }

 // Game objects

 enum AsteroidSize {
   small,
   medium,
   large,
 }

 class Asteroid extends Sprite {
   Point _movementVector;
   AsteroidSize _asteroidSize;
   double _radius;

   double get radius {
     if (_radius != null) return _radius;
     if (_asteroidSize == AsteroidSize.small) _radius = _smlAsteroidRadius;
     else if (_asteroidSize == AsteroidSize.medium) _radius = _medAsteroidRadius;
     else if (_asteroidSize == AsteroidSize.large) _radius = _lrgAsteroidRadius;
     return _radius;
   }

   Asteroid(Texture img, AsteroidSize this._asteroidSize) : super(img) {
     size = new Size(radius * 2.0, radius * 2.0);
     position = new Point(_gameSizeWidth * _rand.nextDouble(), _gameSizeHeight * _rand.nextDouble());
     rotation = 360.0 * _rand.nextDouble();

     _movementVector = new Point(_rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed,
                                 _rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed);

     userInteractionEnabled = true;
   }

   bool handleEvent(SpriteBoxEvent event) {
     if (event.type == "pointerdown") {
       colorOverlay = new Color(0x99ff0000);
     }
     else if (event.type == "pointerup") {
       colorOverlay = null;
     }
     return false;
   }
 }

 class Ship extends Sprite {
   Vector2 _movementVector;
   double _rotationTarget;

   Ship(Texture img) : super(img) {
     _movementVector = new Vector2.zero();
     rotation = _rotationTarget = 270.0;

     // Create sprite
     size = new Size(_shipRadius * 2.0, _shipRadius * 2.0);
     position = new Point(_gameSizeWidth/2.0, _gameSizeHeight/2.0);
   }

   void thrust(double x, double y) {
     _rotationTarget = convertRadians2Degrees(Math.atan2(y, x));
     Vector2 directionVector = new Vector2(x, y).normalize();
     _movementVector.addScaled(directionVector, 1.0);
   }

   void move() {
     position = new Point(position.x + _movementVector[0], position.y + _movementVector[1]);
     _movementVector.scale(0.9);

     rotation = dampenRotation(rotation, _rotationTarget, 0.1);
   }
 }

 class Laser extends Sprite {
   int _frameCount = 0;
   Point _movementVector;
   double radius = 10.0;

   Laser(Texture img, Ship ship) : super(img) {
     size = new Size(20.0, 20.0);
     position = ship.position;
     rotation = ship.rotation + 90.0;
     transferMode = TransferMode.plus;
     double rotRadians = convertDegrees2Radians(rotation);
     _movementVector = pointMult(new Point(Math.sin(rotRadians), -Math.cos(rotRadians)), 10.0);
     _movementVector = new Point(_movementVector.x + ship._movementVector[0], _movementVector.y + ship._movementVector[1]);
   }

   void move() {
     position = pointAdd(position, _movementVector);
     _frameCount++;
   }
 }

 // Background starfield

 class StarField extends Node {
   Texture _img;
   int _numStars;
   List<Point> _starPositions;
   List<double> _starScales;
   List<double> _opacity;

   StarField(this._img, this._numStars) {
     _starPositions = [];
     _starScales = [];
     _opacity = [];

     for (int i  = 0; i < _numStars; i++) {
       _starPositions.add(new Point(_rand.nextDouble() * _gameSizeWidth, _rand.nextDouble() * _gameSizeHeight));
       _starScales.add(_rand.nextDouble());
       _opacity.add(_rand.nextDouble() * 0.5 + 0.5);
     }
   }

   void paint(RenderCanvas canvas) {
     // Setup paint object for opacity and transfer mode
     Paint paint = new Paint();
     paint.setTransferMode(TransferMode.plus);

     double baseScaleX = 32.0 / _img.size.width;
     double baseScaleY = 32.0 / _img.size.height;

     // Draw each star
     for (int i = 0; i < _numStars; i++) {
       Point pos = _starPositions[i];
       double scale = _starScales[i];
       paint.color = new Color.fromARGB((255.0*_opacity[i]).toInt(), 255, 255, 255);

       canvas.save();

       canvas.translate(pos.x, pos.y);
       canvas.scale(baseScaleX * scale, baseScaleY * scale);

       canvas.drawImageRect(_img.image, _img.frame, _img.spriteSourceSize, paint);

       canvas.restore();
     }
   }

   void move(double dx, double dy) {
     for (int i  = 0; i < _numStars; i++) {
       double xPos = _starPositions[i].x;
       double yPos = _starPositions[i].y;
       double scale = _starScales[i];

       xPos += dx * scale;
       yPos += dy * scale;

       if (xPos >= _gameSizeWidth) xPos -= _gameSizeWidth;
       if (xPos < 0) xPos += _gameSizeWidth;
       if (yPos >= _gameSizeHeight) yPos -= _gameSizeHeight;
       if (yPos < 0) yPos += _gameSizeHeight;

       _starPositions[i] = new Point(xPos, yPos);
     }
   }
 }

 class Nebula extends Node {

   Nebula.withImage(Image img) {
     for (int i = 0; i < 2; i++) {
       for (int j = 0; j < 2; j++) {
         Sprite sprt = new Sprite.fromImage(img);
         sprt.pivot = Point.origin;
         sprt.position = new Point(i * _gameSizeWidth - _gameSizeWidth, j * _gameSizeHeight - _gameSizeHeight);
         addChild(sprt);
       }
     }
   }
 }

 // Convenience methods

 Point pointAdd(Point a, Point b) {
   return new Point(a.x+ b.x, a.y + b.y);
 }

 Point pointMult(Point a, double multiplier) {
   return new Point(a.x * multiplier, a.y * multiplier);
 }

 double dampenRotation(double src, double dst, double dampening) {
   double delta = dst - src;
   while (delta > 180.0) delta -= 360;
   while (delta < -180) delta += 360;
   delta *= dampening;

   return src + delta;
 }

 double pointQuickDist(Point a, Point b) {
   double dx = a.x - b.x;
   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }
	part of game;

	const double _steeringThreshold = 0.0;
	const double _steeringMax = 150.0;

	// Random generator
	Math.Random _rand = new Math.Random();

	const double _gameSizeWidth = 1024.0;
	const double _gameSizeHeight = 1024.0;

	const double _shipRadius = 30.0;
	const double _lrgAsteroidRadius = 40.0;
	const double _medAsteroidRadius = 20.0;
	const double _smlAsteroidRadius = 10.0;
	const double _maxAsteroidSpeed = 1.0;

	const int _lifeTimeLaser = 50;

	const int _numStarsInStarField = 150;

	class GameDemoWorld extends NodeWithSize {

	// Images
	Image _imgNebula;

	SpriteSheet _spriteSheet;

	// Inputs
	double _joystickX = 0.0;
	double _joystickY = 0.0;
	bool _fire;

	Node _gameLayer;

	Ship _ship;
	List<Asteroid> _asteroids = [];
	List<Laser> _lasers = [];
	StarField _starField;
	Nebula _nebula;

	GameDemoWorld(ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {

	// Fetch images
	_imgNebula = images["res/nebula.png"];

	_gameLayer = new Node();
	this.addChild(_gameLayer);

	// Add some asteroids to the game world
	for (int i = 0; i < 5; i++) {
	addAsteroid(AsteroidSize.large);
	}
	for (int i = 0; i < 5; i++) {
	addAsteroid(AsteroidSize.medium);
	}

	// Add ship
	addShip();

	// Add starfield
	_starField = new StarField(_spriteSheet["star.png"], _numStarsInStarField);
	_starField.zPosition = -2.0;
	addChild(_starField);

	// Add nebula
	addNebula();

	userInteractionEnabled = true;
	handleMultiplePointers = true;
	}

	// Methods for adding game objects

	void addAsteroid(AsteroidSize size, [Point pos]) {
	Asteroid asteroid = new Asteroid(_spriteSheet["asteroid_big_1.png"], size);
	asteroid.zPosition = 1.0;
	if (pos != null) asteroid.position = pos;
	_gameLayer.addChild(asteroid);
	_asteroids.add(asteroid);
	}

	void addShip() {
	Ship ship = new Ship(_spriteSheet["ship.png"]);
	ship.zPosition = 10.0;
	_gameLayer.addChild(ship);
	_ship = ship;
	}

	void addLaser() {
	Laser laser = new Laser(_spriteSheet["laser.png"], _ship);
	laser.zPosition = 8.0;
	_lasers.add(laser);
	_gameLayer.addChild(laser);
	}

	void addNebula() {
	_nebula = new Nebula.withImage(_imgNebula);
	_gameLayer.addChild(_nebula);
	}

	void update(double dt) {
	// Move asteroids
	for (Asteroid asteroid in _asteroids) {
	asteroid.position = pointAdd(asteroid.position, asteroid._movementVector);
	}

	// Move lasers and remove expired lasers
	for (int i = _lasers.length - 1; i >= 0; i--) {
	Laser laser = _lasers[i];
	laser.move();
	if (laser._frameCount > _lifeTimeLaser) {
	laser.removeFromParent();
	_lasers.removeAt(i);
	}
	}

	// Apply thrust to ship
	if (_joystickX != 0.0 \|\| _joystickY != 0.0) {
	_ship.thrust(_joystickX, _joystickY);
	}

	// Move ship
	_ship.move();

	// Check collisions between asteroids and lasers
	for (int i = _lasers.length -1; i >= 0; i--) {
	// Iterate over all the lasers
	Laser laser = _lasers[i];

	for (int j = _asteroids.length - 1; j >= 0; j--) {
	// Iterate over all the asteroids
	Asteroid asteroid = _asteroids[j];

	// Check for collision
	if (pointQuickDist(laser.position, asteroid.position) < laser.radius + asteroid.radius) {
	// Remove laser
	laser.removeFromParent();
	_lasers.removeAt(i);

	// Add asteroids
	if (asteroid._asteroidSize == AsteroidSize.large) {
	for (int a = 0; a < 3; a++) addAsteroid(AsteroidSize.medium, asteroid.position);
	}
	else if (asteroid._asteroidSize == AsteroidSize.medium) {
	for (int a = 0; a < 5; a++) addAsteroid(AsteroidSize.small, asteroid.position);
	}

	// Remove asteroid
	asteroid.removeFromParent();
	_asteroids.removeAt(j);
	break;
	}
	}
	}

	// Move objects to center camera and warp objects around the edges
	centerCamera();
	warpObjects();
	}

	void centerCamera() {
	const cameraDampening = 0.1;
	Point delta = new Point(_gameSizeWidth/2 - _ship.position.x, _gameSizeHeight/2 - _ship.position.y);
	delta = pointMult(delta, cameraDampening);

	for (Node child in _gameLayer.children) {
	child.position = pointAdd(child.position, delta);
	}

	// Update starfield
	_starField.move(delta.x, delta.y);
	}

	void warpObjects() {
	for (Node child in _gameLayer.children) {
	if (child.position.x < 0) child.position = pointAdd(child.position, new Point(_gameSizeWidth, 0.0));
	if (child.position.x >= _gameSizeWidth) child.position = pointAdd(child.position, new Point(-_gameSizeWidth, 0.0));
	if (child.position.y < 0) child.position = pointAdd(child.position, new Point(0.0, _gameSizeHeight));
	if (child.position.y >= _gameSizeHeight) child.position = pointAdd(child.position, new Point(0.0, -_gameSizeHeight));
	}
	}

	// Handling controls

	void controlSteering(double x, double y) {
	_joystickX = x;
	_joystickY = y;
	}

	void controlFire() {
	addLaser();
	}

	// Handle pointer events

	int _firstPointer = -1;
	int _secondPointer = -1;
	Point _firstPointerDownPos;

	bool handleEvent(SpriteBoxEvent event) {
	Point pointerPos = convertPointToNodeSpace(event.boxPosition);
	int pointer = event.pointer;

	switch (event.type) {
	case 'pointerdown':
	if (_firstPointer == -1) {
	// Assign the first pointer
	_firstPointer = pointer;
	_firstPointerDownPos = pointerPos;
	}
	else if (_secondPointer == -1) {
	// Assign second pointer
	_secondPointer = pointer;
	controlFire();
	}
	else {
	// There is a pointer used for steering, let's fire instead
	controlFire();
	}
	break;
	case 'pointermove':
	if (pointer == _firstPointer) {
	// Handle turning control
	double joystickX = 0.0;
	double deltaX = pointerPos.x - _firstPointerDownPos.x;
	if (deltaX > _steeringThreshold \|\| deltaX < -_steeringThreshold) {
	joystickX = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
	if (joystickX > 1.0) joystickX = 1.0;
	if (joystickX < -1.0) joystickX = -1.0;
	}

	double joystickY = 0.0;
	double deltaY = pointerPos.y - _firstPointerDownPos.y;
	if (deltaY > _steeringThreshold \|\| deltaY < -_steeringThreshold) {
	joystickY = (deltaY - _steeringThreshold)/(_steeringMax - _steeringThreshold);
	if (joystickY > 1.0) joystickY = 1.0;
	if (joystickY < -1.0) joystickY = -1.0;
	}

	controlSteering(joystickX, joystickY);
	}
	break;
	case 'pointerup':
	case 'pointercancel':
	if (pointer == _firstPointer) {
	// Un-assign the first pointer
	_firstPointer = -1;
	_firstPointerDownPos = null;
	controlSteering(0.0, 0.0);
	}
	else if (pointer == _secondPointer) {
	_secondPointer = -1;
	}
	break;
	default:
	break;
	}
	return true;
	}
	}

	// Game objects

	enum AsteroidSize {
	small,
	medium,
	large,
	}

	class Asteroid extends Sprite {
	Point _movementVector;
	AsteroidSize _asteroidSize;
	double _radius;

	double get radius {
	if (_radius != null) return _radius;
	if (_asteroidSize == AsteroidSize.small) _radius = _smlAsteroidRadius;
	else if (_asteroidSize == AsteroidSize.medium) _radius = _medAsteroidRadius;
	else if (_asteroidSize == AsteroidSize.large) _radius = _lrgAsteroidRadius;
	return _radius;
	}

	Asteroid(Texture img, AsteroidSize this._asteroidSize) : super(img) {
	size = new Size(radius * 2.0, radius * 2.0);
	position = new Point(_gameSizeWidth * _rand.nextDouble(), _gameSizeHeight * _rand.nextDouble());
	rotation = 360.0 * _rand.nextDouble();

	_movementVector = new Point(_rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed,
	_rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed);

	userInteractionEnabled = true;
	}

	bool handleEvent(SpriteBoxEvent event) {
	if (event.type == "pointerdown") {
	colorOverlay = new Color(0x99ff0000);
	}
	else if (event.type == "pointerup") {
	colorOverlay = null;
	}
	return false;
	}
	}

	class Ship extends Sprite {
	Vector2 _movementVector;
	double _rotationTarget;

	Ship(Texture img) : super(img) {
	_movementVector = new Vector2.zero();
	rotation = _rotationTarget = 270.0;

	// Create sprite
	size = new Size(_shipRadius * 2.0, _shipRadius * 2.0);
	position = new Point(_gameSizeWidth/2.0, _gameSizeHeight/2.0);
	}

	void thrust(double x, double y) {
	_rotationTarget = convertRadians2Degrees(Math.atan2(y, x));
	Vector2 directionVector = new Vector2(x, y).normalize();
	_movementVector.addScaled(directionVector, 1.0);
	}

	void move() {
	position = new Point(position.x + _movementVector[0], position.y + _movementVector[1]);
	_movementVector.scale(0.9);

	rotation = dampenRotation(rotation, _rotationTarget, 0.1);
	}
	}

	class Laser extends Sprite {
	int _frameCount = 0;
	Point _movementVector;
	double radius = 10.0;

	Laser(Texture img, Ship ship) : super(img) {
	size = new Size(20.0, 20.0);
	position = ship.position;
	rotation = ship.rotation + 90.0;
	transferMode = TransferMode.plus;
	double rotRadians = convertDegrees2Radians(rotation);
	_movementVector = pointMult(new Point(Math.sin(rotRadians), -Math.cos(rotRadians)), 10.0);
	_movementVector = new Point(_movementVector.x + ship._movementVector[0], _movementVector.y + ship._movementVector[1]);
	}

	void move() {
	position = pointAdd(position, _movementVector);
	_frameCount++;
	}
	}

	// Background starfield

	class StarField extends Node {
	Texture _img;
	int _numStars;
	List<Point> _starPositions;
	List<double> _starScales;
	List<double> _opacity;

	StarField(this._img, this._numStars) {
	_starPositions = [];
	_starScales = [];
	_opacity = [];

	for (int i = 0; i < _numStars; i++) {
	_starPositions.add(new Point(_rand.nextDouble() * _gameSizeWidth, _rand.nextDouble() * _gameSizeHeight));
	_starScales.add(_rand.nextDouble());
	_opacity.add(_rand.nextDouble() * 0.5 + 0.5);
	}
	}

	void paint(RenderCanvas canvas) {
	// Setup paint object for opacity and transfer mode
	Paint paint = new Paint();
	paint.setTransferMode(TransferMode.plus);

	double baseScaleX = 32.0 / _img.size.width;
	double baseScaleY = 32.0 / _img.size.height;

	// Draw each star
	for (int i = 0; i < _numStars; i++) {
	Point pos = _starPositions[i];
	double scale = _starScales[i];
	paint.color = new Color.fromARGB((255.0*_opacity[i]).toInt(), 255, 255, 255);

	canvas.save();

	canvas.translate(pos.x, pos.y);
	canvas.scale(baseScaleX * scale, baseScaleY * scale);

	canvas.drawImageRect(_img.image, _img.frame, _img.spriteSourceSize, paint);

	canvas.restore();
	}
	}

	void move(double dx, double dy) {
	for (int i = 0; i < _numStars; i++) {
	double xPos = _starPositions[i].x;
	double yPos = _starPositions[i].y;
	double scale = _starScales[i];

	xPos += dx * scale;
	yPos += dy * scale;

	if (xPos >= _gameSizeWidth) xPos -= _gameSizeWidth;
	if (xPos < 0) xPos += _gameSizeWidth;
	if (yPos >= _gameSizeHeight) yPos -= _gameSizeHeight;
	if (yPos < 0) yPos += _gameSizeHeight;

	_starPositions[i] = new Point(xPos, yPos);
	}
	}
	}

	class Nebula extends Node {

	Nebula.withImage(Image img) {
	for (int i = 0; i < 2; i++) {
	for (int j = 0; j < 2; j++) {
	Sprite sprt = new Sprite.fromImage(img);
	sprt.pivot = Point.origin;
	sprt.position = new Point(i * _gameSizeWidth - _gameSizeWidth, j * _gameSizeHeight - _gameSizeHeight);
	addChild(sprt);
	}
	}
	}
	}

	// Convenience methods

	Point pointAdd(Point a, Point b) {
	return new Point(a.x+ b.x, a.y + b.y);
	}

	Point pointMult(Point a, double multiplier) {
	return new Point(a.x * multiplier, a.y * multiplier);
	}

	double dampenRotation(double src, double dst, double dampening) {
	double delta = dst - src;
	while (delta > 180.0) delta -= 360;
	while (delta < -180) delta += 360;
	delta *= dampening;

	return src + delta;
	}

	double pointQuickDist(Point a, Point b) {
	double dx = a.x - b.x;
	double dy = a.y - b.y;
	if (dx < 0.0) dx = -dx;
	if (dy < 0.0) dy = -dy;
	if (dx > dy) {
	return dx + dy/2.0;
	}
	else {
	return dy + dx/2.0;
	}
	}