»»»Object Collision Detection with Flash 5

One of the core prerequisites for any game built with Macromedia’s Flash is the ability to hit and react to other objects on the stage. This is more often referred to as “collision detection,” that is when you hit something you get hit back. Your basic physics class.

In this article you will learn how to have movies in Flash collide with other movies on the stage. To help demonstrate this more completely we will be using the game BreakOut as an example, as shown in Figure A.

Figure A: BreakOut Game

BreakOut has all of the basic features needed for collision detection. By the end of this article you will know how to do the following:

o        Understand the principles of collision detection

o        Define, programmatically, the area of a game

o        Define, programmatically, how movies should react when they are hit.

You can download the source code for this game from the Element K Journal’s Web site.

The Game
Breakout is a 1976 classic. First appearing on the Atari and then receiving the honors of millions of clones. The basic foundation for the game is to bat a ball around a screen and hit blocks. Each block you hit gives you points. When you have hit all of the blocks you go onto the next screen. You have three lives. When all of the lives are gone, the game ends.

The main object of the game is to send a ball bouncing around the screen. To do this you need to define the area of the screen, what the ball to be bounced is and the paddle from which the ball will be bounced from.

Area of the Game
The first step is to define where the ball will be bouncing. The size of the movie for the BreakOut game is 400px x 500px. This defines the size of the movie, not the area of the game. The game area has to be defined programmatically with ActionScript. The first scripts that will be added will be in Frame 1 of the movie. We start out by creating a variable, called GamePlayArea, with the values calculated in an array. It sounds complicated, but here is what it looks like:

GamePlayArea = [0, 0, 400, 500];

Essentially, what we are doing is telling Flash that the game will cover an area 400 px x 500 px from the top left hand corner of the screen. As we move through the game code, we can reference the Variable name GamePlayArea if we need to know the coordinates of the area of the game.

The Ball
With the game play area now defined, we need to add the ball that will bounce around the stage. It is important that the ball is referenced correctly, other wise the code will not work. The first thing to drag onto the stage is an instance of the movie clip ball. We are going to name the instance of the ball “ball” as shown in  Figure B.

Figure B: The instance of the ball is name “Ball” By naming the instance we can reference it in our script. With this done, we can now begin adding the ActionScript that will bounce the ball. All of the script that controls the ball is placed in the first frame of the movie. The first script you will need to write will control how the ball moves. The script is is:

newBall.prototype.moveBall = function () {

     this.MovieClip._x += this.deltaX;

     this.MovieClip._y += this.deltaY;

     //check if ball hits top of screen

     if (this.MovieClip._y<0) {

           this.flipY();

     }

     if (this.MovieClip._x<GamePlayArea[0] || this.MovieClip._x>GamePlayArea[2]) {

           this.flipX();

     }

}

     if (this.MovieClip._x<GamePlayArea[0] || this.MovieClip._x>GamePlayArea[2]) {

           this.flipX();

     }

Here the movie instance “ball” knows where the area of the game is by referencing the variable “GamePlayArea.” Earlier we told Flash that the game area is only 400x500. The ball will now bounce off an imaginary wall that is 400x500.

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Until next time.. Have fun
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Written by Yury Rush Copyright (©) 2002 motionrush media labs (link)

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