Spent a few hours this past weekend sqeezing every last bit of efficiency out of a starfield generator for Space Survivor.  Fortunately, this was made extremely easy with Richard Lord’s very fine Flint Particle System.

The basic concept:

• Set up 4 particle emitters just outside the 4 edges of the screen
• Feed the Starfield a velocity vector from your ship’s movement engine
• use the velocity vector to give each generated particle a velocity opposite that of the ship
• use the built-in garbage collector of the Flint Particle System to destroy stars that go off screen

There are still some issues with it, like pre-filling the screen with stars, but here’s a demo of it in action.  Code is below the fold:

To use the starfield generator, simply ad the starfield to your stage or a clip with:

_starField = new StarField(_clip.stage, _width, _height, 0, 0);
_clip.addChild(_starField.renderer);
_starField.start();

Where _width and _height are the width and height of your viewport.
And then in your game loop:

_starField.updateTargetVelocity(velX, velY);

Where velX and velY are the X and Y components of the velocity vector of your Object in Space.
And finally, here is the Starfield Class:

package SpaceSurvivor.starfields
{
	import flash.display.Stage;
	import flash.geom.Point;

	import org.flintparticles.actions.*;
	import org.flintparticles.counters.*;
	import org.flintparticles.displayObjects.Dot;
	import org.flintparticles.emitters.Emitter;
	import org.flintparticles.initializers.*;
	import org.flintparticles.renderers.*;
	import org.flintparticles.zones.*;

	public class StarField extends Emitter
	{
		private var _width:uint;
		private var _height:uint;
		private var _stage:Stage;
		private var _targetVelocityAction:TargetVelocity;
		private var _mult:int;
		private var _starCount:Steady;

		private var _isPaused:Boolean = false;

		public function StarField(stage, width, height, velX, velY)
		{
			_width = width;
			_height = height;
			_stage = stage;

			renderer = new DisplayObjectRenderer();
			_starCount = new Steady(50, 100);
			counter = _starCount;

		    addInitializer(new ImageClass(Dot, 1, 0x777777));
		    var multi:MultiZone = new MultiZone();
		    multi.addZone(new LineZone(new Point(-5,-5), new Point(_width+5,-5)));
		    multi.addZone(new LineZone(new Point(_width+5,-5), new Point(_width+5, _height+5)));
		    multi.addZone(new LineZone(new Point(_width+5, _height+5), new Point(-5, _height+5)));
		    multi.addZone(new LineZone(new Point(-5, _height+5), new Point(-5,-5)));

			addInitializer(new Position(multi));
		    addInitializer(new ScaleInit( 0.5, 1.5 ) );

			_targetVelocityAction = new TargetVelocity(velX, velY, 10);
			addAction(_targetVelocityAction);
			addAction(new Move());
		    addAction(new DeathOffStage());
		}

		public function updateTargetVelocity(_velX, _velY):void
		{
			if(_velX == 0 && _velY == 0) {
				pause();
				_isPaused = true;
			} else if (_isPaused) {
				resume();
				_isPaused = false;
			}
			_targetVelocityAction.targetVelocityX = -_velX;
			_targetVelocityAction.targetVelocityY = -_velY;
		}
	}
}