Collision detection for loop problem, only one array item tested? - actionscript-3

Question

[I apologise if this isn't really an in depth question, but I wanted to solve this once and for all]
I was trying to get look into quadtrees, but already ran into trouble getting collision detection without any optimization working properly. Did a search and found a pretty neat example:
http://wonderfl.net/c/kyLx
(onenterframeC part mostly)
Trying to imitate this, it won't work the same.
Only some collisions are detected between particular objects. When the objects are not moving it seems to work alot better for some reason.
I really can't figure out whats the problem, the code is essentially the same as the sample. I did not blindy copy pasted it, I understands whats happening except for this part:
if (j <= i)
continue;
J would never become bigger that I right? The line also completely removes any working collisions for me.
Here is what I did:
[view result here: http://martinowullems.com/collision.swf
Main class
package
{
import com.martino.objects.Square;
import com.martino.world.TestWorld;
import flash.display.MovieClip;
import flash.events.Event;
import net.hires.debug.Stats;
/**
* ...
* #author Martino Wullems
*/
public class CollisionTest extends MovieClip
{
var world:TestWorld;
public function CollisionTest()
{
addEventListener(Event.ADDED_TO_STAGE, onStage);
}
private function onStage(e:Event):void
{
removeEventListener(Event.ADDED_TO_STAGE, onStage);
SetupWorld();
addChild(new Stats());
}
private function SetupWorld():void
{
world = new TestWorld();
addChild(world);
addObjects(50);
}
private function addObjects(amount:int):void
{
for (var i:int = 0; i < amount; ++i) {
var square:Square = new Square(14);
world.addObject(square);
square.x = Math.random() * stage.stageWidth;
square.y = Math.random() * stage.stageHeight;
square.speedX = (Math.random() * 1) + 1;
square.speedY = (Math.random() * 1) + 1;
}
}
}
}
TestWorld
package com.martino.world
{
import com.martino.objects.Ball;
import com.martino.objects.CollisionObject;
import flash.display.MovieClip;
import flash.events.Event;
/**
* ...
* #author Martino Wullems
*/
public class TestWorld extends MovieClip
{
public var objects:Array;
public function TestWorld()
{
initWorld();
}
private function initWorld():void
{
objects = new Array();
addEventListener(Event.ENTER_FRAME, loopWorld);
}
private function loopWorld(e:Event):void
{
for (var i:int = 0; i < objects.length; i++) {
MoveObject(objects[i]);
CheckCollision(i, objects[i]);
}
}
private function CheckCollision(i:int, object:CollisionObject):void
{
//for (var j:int = i + 1; i < objects.length; j++) {
for (var j:int = 0; j < objects.length; j++) {
//if (j <= i)
//continue;
var objectB:CollisionObject = objects[j];
//hittest
if (object.hitTestObject(objectB)) {
object.isHit = true;
objectB.isHit = true;
}else {
object.isHit = false;
objectB.isHit = false;
}
/////////////////
// CHECK X Y //
////////////////
/*if (object.x + object.width < objectB.x) {
} else if (object.x > objectB.x + objectB.width) {
object.isHit = objectB.isHit = false;
} else if (object.y + object.height < objectB.y) {
object.isHit = objectB.isHit = false;
} else if (object.y > objectB.y + objectB.height) {
object.isHit = objectB.isHit = false;
} else {
object.isHit = objectB.isHit = true;
}*/
object.debugDraw();
objectB.debugDraw();
}
}
private function MoveObject(object:CollisionObject):void
{
object.x += object.speedX;
object.y += object.speedY;
////////////////////
//check boundaries//
////////////////////
if (object.x > stage.stageWidth)
{
object.speedX *= -1;
}else if (object.x < 0)
{
object.speedX *= -1;
}else if (object.y > stage.stageHeight)
{
object.speedY *= -1;
}else if (object.y < 0)
{
object.speedY *= -1;
}
}
public function addObject(object:CollisionObject):void
{
objects.push(object);
addChild(object);
}
}
}
CollisionObject
package com.martino.objects
{
import flash.display.Sprite;
import flash.events.MouseEvent;
/**
* ...
* #author Martino Wullems
*/
public class CollisionObject extends Sprite
{
public var size:int;
public var speedX:int = 0;
public var speedY:int = 0;
public var graphic:Sprite;
var sleeping:Boolean = false;
public var isHit:Boolean = false;
public function CollisionObject()
{
addEventListener(MouseEvent.MOUSE_DOWN, grab);
addEventListener(MouseEvent.MOUSE_UP, letGo);
}
private function grab(e:MouseEvent):void
{
startDrag();
speedX = 0;
speedY = 0;
}
private function letGo(e:MouseEvent):void
{
stopDrag();
}
public function Collision():void{
}
//////////////////////
// setter and getter//
//////////////////////
public function set isHit(value:Boolean):void {
_isHit = value;
graphic.visible = _isHit;
hitGraphic.visible = !_isHit;
}
public function get isHit():Boolean {
return _isHit;
}
}
}
Square
package com.martino.objects
{
import flash.display.Sprite;
/**
* ...
* #author Martino Wullems
*/
public class Square extends CollisionObject
{
public var hitGraphic:Sprite;
public function Square(Size:int)
{
size = Size;
drawSquare();
}
private function drawSquare():void
{
graphic = new Sprite();
graphic.graphics.beginFill(0xFF0000);
graphic.graphics.drawRect(0, 0, size, size);
graphic.graphics.endFill();
addChild(graphic);
hitGraphic = new Sprite();
hitGraphic.graphics.beginFill(0x0066FF);
hitGraphic.graphics.drawRect(0, 0, size, size);
hitGraphic.graphics.endFill();
addChild(hitGraphic);
hitGraphic.visible = false;
}
override public function Collision():void {
trace("I collided with a friend (inside joke)");
}
public override function debugDraw():void {
if (isHit) {
graphic.visible = false;
hitGraphic.visible = true;
}else {
graphic.visible = true;
hitGraphic.visible = false;
}
}
}
}
Any help would greatly be appreciated, want to get further on this !
EDIT: Changed some stuff, there is progress but stuff still is unclear to me !
Changed some things in TestWorld.as:
package com.martino.world
{
import com.martino.objects.Ball;
import com.martino.objects.CollisionObject;
import flash.display.MovieClip;
import flash.events.Event;
/**
* ...
* #author Martino Wullems
*/
public class TestWorld extends MovieClip
{
public var objects:Array;
public function TestWorld()
{
initWorld();
}
private function initWorld():void
{
objects = new Array();
addEventListener(Event.ENTER_FRAME, loopWorld);
}
private function loopWorld(e:Event):void
{
var object:*;
for (var i:int = 0; i < objects.length; i++) {
MoveObject(objects[i]);
//CheckCollision(i);// doesn't work here for some reason [case 1]
}
CheckCollision(0); //[case 2]
}
private function CheckCollision(i:int):void
{
//test collision
for (var i:int = 0; i < objects.length; i++){ //only use in case 2
var elementA:CollisionObject;
var elementB:CollisionObject;
elementA = objects[i];
for (var j:int = i + 1; j < objects.length; j++) {
if (j <= i){
continue; //j resets each I loop and therefor sets collision to false while it could be true
}
elementB = objects[ j ]// as ObjSprite;
if (elementA.hitTestObject(elementB)) {
elementA.isHit = elementB.isHit = true;
}
}
} //[case 2]
}
private function MoveObject(object:CollisionObject):void
{
object.x += object.vx;
object.y += object.vy;
////////////////////
//check boundaries//
////////////////////
if (object.x > stage.stageWidth)
{
object.vx *= -1;
}else if (object.x < 0)
{
object.vx *= -1;
}else if (object.y > stage.stageHeight)
{
object.vy *= -1;
}else if (object.y < 0)
{
object.vy *= -1;
}
object.isHit = false;// where do we check when it isn't colliding? this seems messy!
}
public function addObject(object:CollisionObject):void
{
objects.push(object);
addChild(object);
}
}
}
Also added a setter and getter in collisionobject (so section before the edit).
Not sure why I can't put the checkcollision inside the loop on the enter frame function? (when I do no collisions are shown). And placing "isHit = false" inside moveobjects to reset a check for a hit also seems pretty messy.
I can't seem to find out when the objects aren't colliding to reset them I guess.
Making an else statement on the hittest to check if there is no collision doesn't work, seems logical since there could be collisions with more than just 2 items in the hittestcheck.
Any idea's ?

Answer 1

I had to look at the original source to understand this. It is below, for reference.
This line
if (j <= i) continue;
is a permutation check; it basically makes sure that each combination only gets checked once, instead of multiple times. However, for this, you need to have two For loops - an inner one and an outer one.
You've done the same thing with the following line:
for (var j:int = i + 1; i < objects.length; j++) {
Try using that for loop instead of the one that starts from zero. Leave the "if j <= i" line commented, though. I think that will solve your problem. (Those two statements can't coexist in this loop; if used together, they'll cause the problems you're describing.)
Good luck.
Original source from website:
for (i = 0; i < myrects.length; i++) {
elementA = myrects[ i ] as ObjMyRect;
for (j = 0; j < myrects.length; j++) {
if (j <= i)
continue;
elementB = myrects[ j ] as ObjMyRect;
if (elementA.rect.x + elementA.rect.width < elementB.rect.x) {
} else if (elementA.rect.x > elementB.rect.x + elementB.rect.width) {
} else if (elementA.rect.y + elementA.rect.height < elementB.rect.y) {
} else if (elementA.rect.y > elementB.rect.y + elementB.rect.height) {
} else {
elementA.isHit = elementB.isHit = true;
}
}
}
(For what it's worth, I think that this guy's code actually checks objects against themselves for collision - he should change the "if j <= i" line to "if j < i". You solved this problem with your original for loop.)

Answer 2

Couple of things, after looking at his code, and reviewing your code.
This part IS necessary to make sure you don't waste time going through the loop and re-checking items that have already been compared.
if (j <= i)
continue;
Imagine you have 3 items in the array, and compare item 3 to item 0. Then you compare item 3 to item 1, and item 3, to item 2. THEN you compare item 2 to... item 3? You've already done that. You want to compare item 2 to anything lesser than itself so you avoid duplicates.
Your code is close to his, but you've swapped out his version of the hitTest (which uses position + dimension) with the already-defined hitTestObject. I'm only seeing one hit-test on the first square. Something seems off in that... I'd drop some trace statements in there and see what's up.
Last, when you uncomment his code, does it work?