Optimizing collision detection in Actionscript 3.

Question

I'm trying to make my first game in Actionscript 3. Its a simple artillery game where you have a gun at bottom left corner and AIs flying across the screen which you have to shoot down. Now, in a classing shooter game how this is achieved is either by firing a bullet from the gun and doing a hitTest of that bullet with the AI movieClip or by simply removing(or playing the death animation) the AI movieclip. But here's the problem. I dont wanna do either of them. I want to show a gun with a rapid fire rate and hence no bullets. And I also don't want the player to click exactly ON the AI movieclip to destroy them (just the gun/turret pointing at the correct direction should be enough). So here's what I tried.

This image should help picture the whole thing.

1. Tried attaching a 'line' to the turret and checking for collision with AI clip. This obviously failed because of the bounding box problem when the line is slanting.

2. I tried putting multiple tiny movieClips ON the line and iterate through them in a loop and checking for any collision with the AI. This method partially worked, but it was too slow and didn't seem like the 'correct' method.

3. I draw 2 hypothetical lines. One between p1 and p2 and other connecting p3 and p4 (please see the image above). p3----p4's lenght = AI.width and is a horizontal line running right through the center of the AI MovieClip. Upon mouseClick I check for any intersection between these two lines. Now if I have multiple AIs on the screen, Again i need to iterate through each of them, draw a fresh p3-----p4 and check for intersection with p1-----p2.

I'm using method 3 right now and I have a feeling that once I start adding more stuff to the game, framerate will take a hit. I'm sure there should a more simple/optimized method of doing this (maybe something brutally simple). Attaching some relevant code.

shipGame.as

var baseAI:BaseAI;
var AIList:Array= new Array;
public function shipGame():void{
            trace("still connected!!");
            /*
            irrelevant code..
            */
            stage.addEventListener(Event.ENTER_FRAME, updateGame);
            var a:MovieClip = new MovieClip;
            aiTimer = setInterval(StartAI, 2000);
}
public function StartAI():void {
            baseAI = new BaseAI(this);
            AIList.push(baseAI);
}

public function updateGame(event:Event):void    {
            /*
            code to rotate the turret here..
            */

            for (var i:int = 0; i < AIList.length; i++) 
            {
                if (AIList[i].myx < 100) {
                    AIList[i].basicAI.parent.removeChild(AIList[i].basicAI);
                    AIList.splice(i, 1);
                }
            }

            if (mouseDn) {
                basicTurret.gotoAndPlay(2);// Fire animation
                for (var j:int = 0; j < AIList.length; j++) 
                {
                    if ( intersection(AIList[j])) // intersection(bai:BaseAI) returns boolean. Works fine. 
                    {
                        AIList[j].shot = true;
                    }
                }
            }
}

BaseAI.as

var main:shipGame;
var myy:Number;
var myy:Number;
public function BaseAI(doc:shipGame):void{
            main = doc;
            basicAI = new BasicAI(); // BasicAI is a movieClip in the library
            main.addChild(basicAI);
            basicAI.x = 400;
            basicAI.y = Math.abs(Math.random() * (280)) + 20;
            myy = basicAI.y;
            basicAI.addEventListener(Event.ENTER_FRAME, keepMoving);
}

public function keepMoving(evt:Event):void {
        basicAI.x -= 5;
        myx = basicAI.x;

        if (shot && alive) {
            alive = false;
            basicAI.gotoAndPlay(2);
        }
}

Answer 1

All your current methods are essentially relying on Flash's renderer to know where the ships and bullet-lines are. But in your case, since you have a relatively simple geometric relationship between the moving parts, it would be much faster to handle the problem analytically.

Consider the case where a ship is sitting precisely on your gun's line of fire. In this case the gun, ship and bottom of the screen form a right triangle. If we define theta to be the angle between the gun and the bottom of the screen, then the following will hold:

tangent(theta) = ship.y / ship.x

where x and y are measured rightwards and upwards from the gun. Now you can generalize this and use it for your hit testing. When the gun is fired, first find theta. Then for each ship, you can easily determine how far vertically or horizontally the ship is from the line of fire, by doing something like this:

var theta = /*determine theta based on where the mouse was clicked*/
var tanTheta = tangent(theta);
for each (ship) {
    var gunX = ship.y / tanTheta;
    if ( abs(gunX - ship.x) < ship.width/2 ) {
        // a hit occurred
    }
}

That's just pseudocode but hopefully you get the idea. Anyway that's how I would do it - this will scale hugely faster than anything you do with real hit testing, and for the described case it should work just as well.