# Playing Around with Elastic Collisions

##### Tutorial Details
• Difficulty: Intermediate
• Platform: Flash
• Language: AS3
• Software used: Flash Professional CS5
• Estimated Completion Time: 60 mins
This entry is part 6 of 13 in the You Do The Math Session
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In this tutorial we will create a game where the objective is to prevent other objects from colliding with your cursor. We won’t use Flash’s built-in hitTestObject() methods; instead we will write our own collision detection routines.

Republished Tutorial

Every few weeks, we revisit some of our reader's favorite posts from throughout the history of the site. This tutorial was first published in February of 2011.

## Final Result Preview

Let’s take a look at the final result we will be working towards:

## Step 1: Start Off

Create a new Flash file (ActionScript 3.0)

Set the stage dimensions to 500x500px and FPS to 32.

## Step 2: The Ball Class

This class will contain all data related to one ball. A ball has a _mass, a _radius, an _xSpeed and a _ySpeed. So we will make a property for each. In the constructor we pass the mass, the angle and the speed of the ball. Because the class will be linked to a display object we can retrieve the the radius of our ball by dividing the width of the display object by 2. The _xSpeed and _ySpeed can be calculated by using simple sine and cosine functions.

package
{
import flash.display.Stage
import flash.display.Sprite
import flash.events.Event

public class Ball extends Sprite
{
private var _mass:Number = 0
private var _xSpeed:Number = 0
private var _ySpeed:Number = 0

public function Ball(mass:Number = 10.0, angle:Number = Math.PI, speed:Number = 10.0):void
{
this.mass = mass
this.xSpeed = speed*Math.sin(angle)
this.ySpeed = speed*Math.cos(angle)
}
}
}


For more information on these trigonometric Math.sin() and Math.cos() functions, see this Quick Tip.

## Step 3: Providing Getters and Setters

In our Ball class we provide getters and setters for our properties.

public function get radius():Number
{
}

public function set mass(mass:Number):void
{
this._mass = mass
}

public function get mass():Number
{
return this._mass
}

public function set xSpeed(xSpeed:Number):void
{
this._xSpeed = xSpeed
}

public function get xSpeed():Number
{
return this._xSpeed
}

public function set ySpeed(ySpeed:Number):void
{
this._ySpeed = ySpeed
}

public function get ySpeed():Number
{
return this._ySpeed
}


## Step 4: Update Function

This function updates the x and y properties of our ball according to the _xSpeed and _ySpeed. We’ll implement this function in our Ball class.

public function update():void
{
this.x += _xSpeed
this.y += _ySpeed
}


## Step 5: The Completed Class

We’ll finish up our Ball class in this step.

package
{
import flash.display.Stage
import flash.display.Sprite
import flash.events.Event

public class Ball extends Sprite
{
private var _mass:Number = 0
private var _xSpeed:Number = 0
private var _ySpeed:Number = 0

public function Ball(mass:Number = 10.0, angle:Number = Math.PI, speed:Number = 10.0):void
{
this.mass = mass
this.xSpeed = speed*Math.sin(angle)
this.ySpeed = speed*Math.cos(angle)
}

{
}

public function set mass(mass:Number):void
{
this._mass = mass
}

public function get mass():Number
{
return this._mass
}

public function set xSpeed(xSpeed:Number):void
{
this._xSpeed = xSpeed
}

public function get xSpeed():Number
{
return this._xSpeed
}

public function set ySpeed(ySpeed:Number):void
{
this._ySpeed = ySpeed
}

public function get ySpeed():Number
{
return this._ySpeed
}

public function update():void
{
this.x += _xSpeed
this.y += _ySpeed
}
}
}


## Step 6: Display Objects for our Ball Class

In the source-files I included a start FLA which contains all the library items you need. You can draw them yourself if you want, of course. Make sure your FLA has the following display objects:

(Ed. note: that’s a typo: “ennemyball” should say “enemyball”.)

## Step 7: Linking our Library Objects

The Ball class we just created has to be linked to the enemyball Sprite in the library.

The playerball Sprite must have Ball as base class and PlayerBall as class.

The score movie clip must have a Score class.

## Step 8: The Application Class (Document Class)

The Application class will contain all the game logic. We import all the classes we need. As you can see we’ll be using TweenMax.

Next we define our field variables. The first field variable is the ballPlayer.

Because the base class of our playerball Sprite is Ball we can store this Class in the ballPlayer variable. This makes it easier later on to check for collisions between the ballPlayer and the enemy balls.

The second field variable is an array which will contain all our enemy balls. The third variable is the timer that will be used to perform the main game loop. The fourth and last field is an instance of our Score library object that will be used to display the elapsed game time. In the constructor we call the init() function which I’ll explain in the next step.

package
{
import flash.display.Sprite
import flash.display.Graphics
import flash.events.Event
import flash.events.TimerEvent
import flash.events.MouseEvent
import flash.geom.Matrix
import flash.utils.Timer
import flash.ui.Mouse
import com.greensock.TweenMax
import com.greensock.easing.*

public class Application extends Sprite
{

private var ballPlayer:Ball
private var eballs:Array
private var tmr:Timer
private var score:Score

public function Application():void
{
init()
}
}
}


Don’t forget to link the document class!.

## Step 9: The init() Function

Take a look at this code:

private function init():void
{
ballPlayer = new PlayerBall()
eballs = new Array()
tmr = new Timer(10)
score = new Score()

stage.align = "TL"
stage.scaleMode = "noScale"
Mouse.hide()

setBackground()

score.x = stage.stageWidth/2
score.y = stage.stageHeight/2

}


In the first four lines we initialize our field variables.

Next we make sure our stage is aligned to the top left corner and does not scale.

We hide the mouse cursor. Our cursor will be replaced with the playerball Sprite. Next we call the setBackground function(explained in the next step).

We center our score on the screen and add it to the display list. To update the position of ballPlayer we attach a MouseEvent.MOUSE_MOVE event to the stage.

The updatePlayerBall function (explained in step 11) will handle this MouseEvent. Next we add the ballPlayer to the display list.

The timer will be used to display the game time. We attach a TimerEvent.TIMER listener to our timer, which will trigger the updateTime() function (explained in Step 12) every 10 milliseconds.

Finally, we add a MouseEvent.CLICK to our stage. The startGame function (explained in step 13) will then start our game.

## Step 10: setBackground() Function

This function adds a radial gradient background to the display list. To draw a gradient on a Sprite you must define the type of gradient, the colors you want to use, alpha values of the colors, the ratios (these define the distribution of the colors)and the spread method.

private function setBackground():void
{
var colors:Array = [0xffffff,0xcccccc]
var alphas:Array = [ 1, 1 ]
var ratios:Array = [ 0, 255 ]
var matr:Matrix = new Matrix()
matr.createGradientBox(stage.stageWidth, stage.stageHeight, Math.PI / 2, 0, 0 )
//Start the Gradietn and pass our variables to it
var sprite:Sprite = new Sprite()
//Save typing + increase performance through local reference to a Graphics object
var g:Graphics = sprite.graphics
g.beginGradientFill( type, colors, alphas, ratios, matr, sprMethod )
g.drawRect(0,0,stage.stageWidth,stage.stageHeight)

}


## Step 11: updatePlayerBall() Function

This function updates the position of ballPlayer according to the position of your mouse.

private function updatePlayerBall(e:MouseEvent):void
{
ballPlayer.x = mouseX
ballPlayer.y = mouseY
}


## Step 12: updateTime() Function

We calculate the time in seconds and put it inside the textbox of our score Sprite. Every 5000ms (five seconds) we add a new ball to the game.

private function updateTime(e:TimerEvent):void
{
score.txtScore.text = String(((tmr.currentCount*tmr.delay)/1000).toFixed(2));
if((tmr.currentCount*tmr.delay) % 5000 == 0)
{
}
}


## Step 13: startGame() Function

The game is started by clicking the stage. First we remove the listener for the stage click, so that we can’t start the game serveral times. We add three balls to the game by calling the addBall() function (explained in the next step) three times. We start our timer which will update our game time.

Finally we add an ENTER_FRAME event to our stage. The gameLoop() function (explained in Step 15) will update the position of our enemy balls.

private function startGame(e:MouseEvent):void
{
stage.removeEventListener(MouseEvent.CLICK, startGame)
tmr.start()
}


First we make a new instance of our Ball class. We position the ball randomly on the stage with an alpha of 0 and add it to the display list.

Next we tween the alpha back to 1. (I use TweenMax, it’s included in the source files. You can also use the built-in Flash tween engine.) The second tween isn’t really a tween. It just waits a second and the onComplete function pushes the ball into our eballs array. This way the gameLoop() function (explained in the next step) can handle the rest.

private function addBall():void
{
var ball:Ball = new Ball(10, Math.random()*Math.PI*2, 5)
ball.x = Math.random()*stage.stageWidth
ball.y = Math.random()*stage.stageHeight
ball.alpha = 0
TweenMax.to(ball, 0.5, {alpha:1})
TweenMax.to(ball, 0, {delay: 1, onComplete:function():void{eballs.push(ball)}})
}


## Step 15: gameLoop() Function

Every frame will go through this function.

private function gameLoop(e:Event):void
{
for (var i:uint = 0; i < eballs.length; i++)
{
for (var j:uint = i + 1; j < eballs.length; j++)
{
if (collision(eballs[i], eballs[j]))
{
doCollision(eballs[i], eballs[j])
}
}

if(collision(eballs[i], ballPlayer))
{
endOfGame()
break
}

eballs[i].update()
checkBounds(eballs[i])
}
}


We start by iterating through all of our enemy balls.

The second for-loop checks for collisions between the enemy balls. The loop starts at ‘i + 1′. This way we don’t double check the collisions.

Next we check if the ballPlayer hits the enemy ball. If so, the game is finished. Then we update the position of our enemy ball.

We make sure the balls stay in the game screen by calling the function checkBounds() (explained later on).

## Step 16: collision() Function

This function checks whether any given pair of balls are colliding.

First we calculate the x distance and the y distance between the two balls. Using Pythagoras’s Theorem (see the following diagram) we calculate the absolute distance between them. If the distance is less or equal to the sum of the the radii of the balls we have a collision.

private function collision(ball1:Ball, ball2:Ball):Boolean
{
var xDist:Number = ball1.x - ball2.x
var yDist:Number = ball1.y - ball2.y
var Dist:Number = Math.sqrt(xDist * xDist + yDist * yDist)

}


## Step 17: doCollision() Function

This function will calculate the new x- and y-speeds of the balls according to the speed and angle of the collision. Warning: math ;)

First we calculate the horizontal distance between the two balls and then the vertical distance between the balls. With these distances (and a little more trigonometry) we can calculate the angle between the balls (see diagram).

Next we calculate what I call the magnitude of each ball. (We have an xspeed vector and a yspeed vector; the magnitude is the vector sum of those.) Then we calculate the angle of each ball (similar to the previous angle calculation).

Next we rotate the new x- and y-speeds of each ball. What we are actually doing is rotating the coordinate system. By rotating our axes we have a 1D collision. (See the following diagram).

Newton says that the total amount of kinetic energy in a closed system is constant. Now we use these formulae:

• v1 = (u1*(m1-m2) + 2*m2*u2)/(m1+m2)
• v2 = (u2*(m2-m1) + 2*m1*u1)/(m1+m2)

where:

v1 = final xSpeedBall 1

v2 = final xSpeedBall 2

m1 = mass ball 1

m2 = mass ball 2

u1 = initial speed ball 1

u2 = initial speed ball 2

The y-speeds don’t change as it is a 1D collision.

With these formulae we can calculate the xSpeed and ySpeed of each ball.

Now whe have the new x- and y-speeds in our rotated coordinate system. The last step is to convert everything back to a normal coordinate system. We use Math.PI/2 because the angle between xSpeed and ySpeed must always be 90 degrees (pi/2 radians).

private function doCollision(ball1:Ball, ball2:Ball):void
{
var xDist:Number = ball1.x - ball2.x
var yDist:Number = ball1.y - ball2.y
var collisionAngle:Number = Math.atan2(yDist, xDist)

var magBall1:Number = Math.sqrt(ball1.xSpeed*ball1.xSpeed+ball1.ySpeed*ball1.ySpeed)
var magBall2:Number = Math.sqrt(ball2.xSpeed*ball2.xSpeed+ball2.ySpeed*ball2.ySpeed)

var angleBall1:Number = Math.atan2(ball1.ySpeed, ball1.xSpeed)
var angleBall2:Number = Math.atan2(ball2.ySpeed, ball2.xSpeed)

var xSpeedBall1:Number = magBall1 * Math.cos(angleBall1-collisionAngle)
var ySpeedBall1:Number = magBall1 * Math.sin(angleBall1-collisionAngle)
var xSpeedBall2:Number = magBall2 * Math.cos(angleBall2-collisionAngle)
var ySpeedBall2:Number = magBall2 * Math.sin(angleBall2-collisionAngle)

var finalxSpeedBall1:Number = ((ball1.mass-ball2.mass)*xSpeedBall1+(ball2.mass+ball2.mass)*xSpeedBall2)/(ball1.mass+ball2.mass)
var finalxSpeedBall2:Number = ((ball1.mass+ball1.mass)*xSpeedBall1+(ball2.mass-ball1.mass)*xSpeedBall2)/(ball1.mass+ball2.mass)
var finalySpeedBall1:Number = ySpeedBall1
var finalySpeedBall2:Number = ySpeedBall2

ball1.xSpeed = Math.cos(collisionAngle)*finalxSpeedBall1+Math.cos(collisionAngle+Math.PI/2)*finalySpeedBall1
ball1.ySpeed = Math.sin(collisionAngle)*finalxSpeedBall1+Math.sin(collisionAngle+Math.PI/2)*finalySpeedBall1
ball2.xSpeed = Math.cos(collisionAngle)*finalxSpeedBall2+Math.cos(collisionAngle+Math.PI/2)*finalySpeedBall2
ball2.ySpeed = Math.sin(collisionAngle)*finalxSpeedBall2+Math.sin(collisionAngle+Math.PI/2)*finalySpeedBall2
}


## Step 18: endOfGame() Function

This is run when the game ends.

private function endOfGame():void
{
tmr.stop()
Mouse.show()
stage.removeEventListener(MouseEvent.MOUSE_MOVE, updatePlayerBall)
stage.removeEventListener(Event.ENTER_FRAME, gameLoop)

while(eballs.length > 0)
{
TweenMax.to(eballs[0], 0.5, {scaleX:0, scaleY:0, ease:Bounce.easeOut})
eballs.splice(0,1)
}

TweenMax.to(ballPlayer, 0.5, {scaleX:0, scaleY:0, ease:Bounce.easeOut})
}


First off all we stop the timer. We show the mouse again. Next we remove both the MOUSE_MOVE and ENTER_FRAME event listeners. Finally we make all the balls on the stage invisible.

## Step 19: checkBounds() Function

This function makes sure the balls stay inside the game screen. So if the ball hits the upper or lower side, we reverse the ySpeed. if the ball hits the left or the right side of the screen we reverse the xSpeed. It uses similar logic to the ball collision detection function to check whether the edges of the ball hits an edge of the screen.

private function checkBounds(ball:Ball):void
{
{
ball.xSpeed *= -1
}
{
ball.xSpeed *= -1
}
{
ball.ySpeed *= - 1
}
{
ball.ySpeed *= - 1
}
}


## Step 20: The Complete Application Class

We have completed our Application class. We now have a working game!!!

package
{
import flash.display.Sprite;
import flash.display.Graphics;
import flash.events.Event;
import flash.events.TimerEvent;
import flash.events.MouseEvent;
import flash.geom.Matrix;
import flash.utils.Timer;
import flash.ui.Mouse;
import com.greensock.TweenMax;
import com.greensock.easing.*;
public class Application extends Sprite
{

private var ballPlayer:Ball;
private var eballs:Array;
private var tmr:Timer;
private var score:Score;
public function Application():void
{
init();
}

private function init():void
{
ballPlayer = new PlayerBall();
eballs = new Array();
tmr = new Timer(10);
score = new Score();

stage.align = "TL";
stage.scaleMode = "noScale";
Mouse.hide();

setBackground();

score.x = stage.stageWidth / 2;
score.y = stage.stageHeight / 2;

}

private function setBackground():void
{
var colors:Array = [0xffffff,0xcccccc];
var alphas:Array = [1,1];
var ratios:Array = [0,255];
var matr:Matrix = new Matrix();
matr.createGradientBox(stage.stageWidth, stage.stageHeight, Math.PI / 2, 0, 0 );
//Start the Gradietn and pass our variables to it
var sprite:Sprite = new Sprite();
//Save typing + increase performance through local reference to a Graphics object
var g:Graphics = sprite.graphics;
g.beginGradientFill( type, colors, alphas, ratios, matr, sprMethod );
g.drawRect(0,0,stage.stageWidth,stage.stageHeight);
}

private function updatePlayerBall(e:MouseEvent):void
{
ballPlayer.x = mouseX;
ballPlayer.y = mouseY;
}

private function updateTime(e:TimerEvent):void
{
score.txtScore.text = String(((tmr.currentCount*tmr.delay)/1000).toFixed(2));
if ((tmr.currentCount*tmr.delay) % 5000 == 0)
{
}
}

private function startGame(e:MouseEvent):void
{
stage.removeEventListener(MouseEvent.CLICK, startGame);
tmr.start();
}

{
var ball:Ball = new Ball(10,Math.random() * Math.PI * 2,5);
ball.x = Math.random() * stage.stageWidth;
ball.y = Math.random() * stage.stageHeight;
ball.alpha = 0;
TweenMax.to(ball, 0.5, {alpha:1});
TweenMax.to(ball, 0, {delay: 1, onComplete:function():void{eballs.push(ball)}});
}

private function gameLoop(e:Event):void
{
for (var i:uint = 0; i < eballs.length; i++)
{
for (var j:uint = i + 1; j < eballs.length; j++)
{
if (collision(eballs[i],eballs[j]))
{
doCollision(eballs[i], eballs[j]);
}
}

if (collision(eballs[i],ballPlayer))
{
endOfGame();
break;
}

eballs[i].update();
checkBounds(eballs[i]);
}
}

private function collision(ball1:Ball, ball2:Ball):Boolean
{
var xDist:Number = ball1.x - ball2.x;
var yDist:Number = ball1.y - ball2.y;
var Dist:Number = Math.sqrt(xDist * xDist + yDist * yDist);

{
if (ball1.x < ball2.x)
{
ball1.x -=  2;
ball2.x +=  2;
}
else
{
ball1.x +=  2;
ball2.x -=  2;
}

if (ball1.y < ball2.y)
{
ball1.y -=  2;
ball2.y +=  2;
}
else
{
ball1.y +=  2;
ball2.y -=  2;
}
}

}

private function doCollision(ball1:Ball, ball2:Ball):void
{
var xDist:Number = ball1.x - ball2.x;
var yDist:Number = ball1.y - ball2.y;
var collisionAngle:Number = Math.atan2(yDist,xDist);
var magBall1:Number = Math.sqrt(ball1.xSpeed * ball1.xSpeed + ball1.ySpeed * ball1.ySpeed);
var magBall2:Number = Math.sqrt(ball2.xSpeed * ball2.xSpeed + ball2.ySpeed * ball2.ySpeed);
var angleBall1:Number = Math.atan2(ball1.ySpeed,ball1.xSpeed);
var angleBall2:Number = Math.atan2(ball2.ySpeed,ball2.xSpeed);
var xSpeedBall1:Number = magBall1 * Math.cos(angleBall1 - collisionAngle);
var ySpeedBall1:Number = magBall1 * Math.sin(angleBall1 - collisionAngle);
var xSpeedBall2:Number = magBall2 * Math.cos(angleBall2 - collisionAngle);
var ySpeedBall2:Number = magBall2 * Math.sin(angleBall2 - collisionAngle);
var finalxSpeedBall1:Number = ((ball1.mass-ball2.mass)*xSpeedBall1+(ball2.mass+ball2.mass)*xSpeedBall2)/(ball1.mass+ball2.mass);
var finalxSpeedBall2:Number = ((ball1.mass+ball1.mass)*xSpeedBall1+(ball2.mass-ball1.mass)*xSpeedBall2)/(ball1.mass+ball2.mass);
var finalySpeedBall1:Number = ySpeedBall1;
var finalySpeedBall2:Number = ySpeedBall2;
ball1.xSpeed = Math.cos(collisionAngle) * finalxSpeedBall1 + Math.cos(collisionAngle + Math.PI / 2) * finalySpeedBall1;
ball1.ySpeed = Math.sin(collisionAngle) * finalxSpeedBall1 + Math.sin(collisionAngle + Math.PI / 2) * finalySpeedBall1;
ball2.xSpeed = Math.cos(collisionAngle) * finalxSpeedBall2 + Math.cos(collisionAngle + Math.PI / 2) * finalySpeedBall2;
ball2.ySpeed = Math.sin(collisionAngle) * finalxSpeedBall2 + Math.sin(collisionAngle + Math.PI / 2) * finalySpeedBall2;
}

private function endOfGame():void
{
tmr.stop();
Mouse.show();
stage.removeEventListener(MouseEvent.MOUSE_MOVE, updatePlayerBall);
stage.removeEventListener(Event.ENTER_FRAME, gameLoop);

while (eballs.length > 0)
{
TweenMax.to(eballs[0], 0.5, {scaleX:0, scaleY:0, ease:Bounce.easeOut});
eballs.splice(0,1);
}

TweenMax.to(ballPlayer, 0.5, {scaleX:0, scaleY:0, ease:Bounce.easeOut});
}

private function checkBounds(ball:Ball):void
{
if ((ball.x + ball.radius) > stage.stageWidth)
{
ball.xSpeed *=  -1;
}
if ((ball.x - ball.radius) < 0)
{
ball.xSpeed *=  -1;
}
if ((ball.y + ball.radius) > stage.stageHeight)
{
ball.ySpeed *=  -1;
}
if ((ball.y - ball.radius) < 0)
{
ball.ySpeed *=  -1;
}
}
}
}


## Conclusion

That’s it for this tutorial. Of course you could add the possibility to restart the game, but that shouldn’t be too hard. This basic example of elastic collisions can be used for bigger games like a billiards game or similar.

I hope you liked this tutorial, thanks for reading!

• knalle

Expected something different form that title…
What I thought was interesting i just a tween.

Good beginner tutorial though :)

• http://www.stevendevooght.be Steven Devooght

The collision itself isn’t created by using tweens. The tut covers the mathematics to create an elastic collision.

• Josh

I’m very new to Flash development and tried to walk through this using CS5 without downloading the assets. I had a heck of a time trying to get it to work because I kept getting TypeError: Error #1009 on the stage reference in init().

I finally downloaded the project and dug around a bit. I eventually got it to work by changing the base class of my score symbol to flash.display.Sprite from flash.display.MovieClip. I did this by changing the base class in Symbol properties. I’d love to know why this fixed the problem and if there’s an easier way to convert the Dynamic Text to a sprite.

• http://www.stevendevooght.be Steven Devooght

Setting the base class of your score symbol to Sprite or to MovieClip shouldn’t make a big difference. A Sprite is in fact a MovieClip with only one keyframe. I changed the base class to both Sprite and MovieClip and had no problems running the project. Deleting the score symbol and inserting an new one might fix your problem. Flash CS4 used to have a bug when changing the base class of your library symbols, don’t know if this is still the case in flash CS5.

If you encounter further problems you may always send me your source code so i can look into this a little bit deeper.

• http://www.nitras.be Nit’ras

aha, another belgian author ;)

nice tut, its great to see how simple math can offer a batch of different end results.

• http://www.martincourchesne.com Martin

Nice !!!!

60,53 !!!!

;-)

• MrBond

thanks for this tutorial it’s very helpful

simple, but well written, thank you for sharing :-)

• Dav1d

my timing was 68.64 secs

• http://www.archipelago-of-strategy-games.com/ Stef

The code is well presented: big fonts, nice colors… That’s great. Thanks.

• http://active.tutsplus.com John Grafh

hi there, can someone tell me how to add a restart button or even for it to restart on click after you have died. Thanks

• http://www.bina.com.tr söve

thanks for this tutorial it’s very helpful

• Tyler

60.64 HA!!!

• http://www.waterfall-home.com Lsmith

I got 88.77! The secret to doing well in this game is your interface. Try doing it with a pen pad!

• Dude

Or you can just drag the cursor out of the stage. My record? Still counting…..

• Shantanu Salvi

Thanks a lot Steven for this great and wonderful tutorial. It saved my day :)

• sh

88;P

• http://www.mycrazygames.com Elizabeth

Very well presented and easy to understand tutorial.

Very nice Tutorial Steven, thanks for that.

But i ran into a problem:
Because i don’t use the flash IDE, and only work in actionscript-only enviroment, i can’t add/draw/define symbols like the playerball etc.
So i tried something different, i created 2 classes only, the Ball-class and the PlayerBall-Class(which inherits the Ball-class). in the constructor of the Ball class i draw a circle and fill it with color. The balls all display on-stage and zap around like in your demo-swf, the only problem is : the collison checks don’t register anything, not between enemyballs, and not between an enemy and the player.

I can’t figure out how to solve this, so if anybody has an idea, you’ll have my eternal gratitude. and no, downloading Flash CS3/4/5/6 is not the solution, or well, it is “a” solution that i don’t want to use.