The elusive hitTest explained

[Ed Mack]

Back by popular demand!

HitTest is a very useful, and can be used for just about anything (like ductape really). There are two main forms that it takes:

if(this.hitTest(another_clip)){
 // Something
}

This just tests the bounding boxes of this, and another_clip. If they hit, then the contents of the if will run.

if(this.hitTest(x, y, true)){
 // Something
}

This tests the point x,y against the contents of this, the actual shape and not the bounding box. The true is what makes it test against the shape and not the bounding box.

Anyone else want to continue?

(if it's an irrelevant post, it'll be edited out as this is a guide sticky, not just a general bantering one )

[strangelife2k]

I am going to show you how to have a hitTest on your badguy so that if your hero touches it, it will decrease your life.

1.Goto your hero's instance box and name it "player"

2.Add a dynamic text box on your scene, and set it's variable to "life".

3.Add this code to Frame 1

life = 100;

Edit the life if you wish

4.Bring up the object actions panel for your enemy and add these actions:

onClipEvent (enterFrame) {
    if (this.hitTest(_root.player)) {
        _root.life -= 5;
        if (_root.life == 0) {
            _root.gotoAndStop(2);
        }
    }
}

These actions cause the variable "life" on _root (the one you set the dynamic text field to display) to go down by 5 each time the player and the enemy collide. It also makes the main timeline goto frame two, if the player runs out of life. You should put a gameover screen on frame two of _root.

There you go, test it out

[ed: Just tweaking a few readability things, thanks very much for this ]

[BlinkOk]

[Ed Mack]

Thanks! Is there much more that we can do?

Here's code I'm quite fond of using for moving the player. It requires a clip on _root, called map containing the areas you're not allowed to walk on (It basically tests the bounding box of the object against an actual shape):

onClipEvent(load){
	s = 5;
	b = this.getBounds(this);

	function move(x,y){
		if(!_root.hit.hitTest(_x+x+b.xmin+1, _y+y+b.ymin+1, true)){
			if(!_root.hit.hitTest(_x+x+b.xmax-1, _y+y+b.ymin+1, true)){
				if(!_root.hit.hitTest(_x+x+b.xmin+1, _y+y+b.ymax-1, true)){
					if(!_root.hit.hitTest(_x+x+b.xmax-1, _y+y+b.ymax-1, true)){
						_x += x;
						_y += y;
					}
				}
			}
		}
	}
}

onClipEvent(enterFrame){
	if(Key.isDown(Key.UP)){
		move(0,-s);
	}
	if(Key.isDown(Key.DOWN)){
		move(0,s);
	}
	if(Key.isDown(Key.LEFT)){
		move(-s,0);
	}
	if(Key.isDown(Key.RIGHT)){
		move(s,0);
	}
}

[ViRGo_RK]

if(this.hitTest(x, y, true)){
 // Something
}

This tests the point x,y against the contents of this, the actual shape and not the bounding box. The true is what makes it test against the shape and not the bounding box.

This would be useful for, say, when leaving a level. Like your hero. once he hits a door or something, (this is for art-based), you would just set the right x and y coords and poof! Perfect for art based...

[mosterdfles_flash]

Ah well i'll gonna be a part of this to

This _x, _y, true thingy can also be used for games like this:
http://members.lycos.nl/saphua/games/newBouncy.swf

It will walk on the hill instead of on the bounding box

There are loads of files here on flashkit wich explain this very well (atleast bether than my [poor] english)

[NeotiK]

this works really nicely for art-based RPG's where you go from room to room, but is there any way to modify the code to make a large outdoor map that will start to scroll up/down/left/right as soon as the "hero" gets to the midpoint of the screen? If you still don't know what I'm talking about here's an example someone did that I'm trying to implement into my RPG engine, but i'm stumped on how to do it using the current script setup: http://www.geocities.com/mclelun/rpg.html

Any help would be appreciated. Thanx.

[flashGaul]

HI,
I just want to detect collision between two circles not with there bounding boxes but there actual shapes using shapeFlag property of flash.

I came acros this thread actual wherein the discussion is alrady on.
Anyway
we all know this is right
onCliEvent(enterFrame){
if(hitTest(_root._xmouse,_root._ymouse,true)){
trace(Hit is on);
}
}

This works fine with the _xmouse and _ymouse property.

But what If I want to do this witha movie clip but checking the actual shape of the movieclip not the bounding box.

something like :
onCliEvent(enterFrame){
if(hitTest(_root.mov._x,_root.mov._y,true)){
trace(Hit is on);
}
}

This also works but it works only when the center of the "mov" clip which is basically a circle is hit by the host movie clip i:e _x and _y of mov.

I want it to happen when the mov actaully collides iwth the host movielcip

So I treid

onCliEvent(enterFrame){
if(hitTest(_root.mov._x-_root.mov._width/2,_root.mov._x-
_root.mov._width/2,_root.mov._y-_root.mov._height/2,_root.mov._y+
_root.mov._height/2){
trace(Hit is on);
}
}


This is to detect the actual collison of the mov movie clip.
But this isnt happening.

Also I have heard that this type of hitTest method in flash consumes quiet an amount of memory resulting in lower fps.

Pls guide me if I am wrong somewhere.

thx

[Ed Mack]

The easiest way to check if two circles hit, is to work out the distance between the two circle's centers, and check if it less than both circle's radias (?) added together.

[flashGaul]

that worked, that really worked.

thx edmack that was gr8 help.

this is what I tried and got it. The important thing is that I have totally eluded the hitTest function of flash.

onClipEvent (enterFrame) {
x = _x-_root.two._x;
y = _y-_root.two._y;
h = Math.sqrt((x*x)+(y*y));
rad = _width/2;
radTwo = _root.two._width/2;
sumRad = rad+radTwo;
if (sumRad>=h) {
trace("yes");
}
}
onClipEvent(load){
startDrag("", true);
}

two is the other movie clip on the stage.

Infact I am working on a pool game with this help I can really move ahead. I will start posting threads based on this.

thx again.

[Psytrix]

Originally posted by Ed Mack

It requires a clip on _root, called map containing the areas you're not allowed to walk on (It basically tests the bounding box of the object against an actual shape):

Could you explain that code a lil better, im not sure exactly how it works...?

[jwtheiss]

NeotiK, your engine is looking sweet.
Ed Mack, I think someone got an 'A' in geometry.

[joe_m_87]

is there a proper way to do a hitTest that checks if some one is clicking on the object? if not are you supposed to use buttons to do this? i have seen many games where a click alters a MC... (shoot-em-up types obviously)

-joe

[flashGaul]

try this

PHP Code:

onClipEvent(mouseUp){

if(hitTest(_root._xmouse,_root._ymouse)){
//Your statements here;
}
}


Copy Paste the above code in movieclip.

[jwtheiss]

don't need a "this." before the hitTest?
that's something good to know. just implied when inside a movieClip script?

[devnull_2k]

If you do not write "this", ActionScript assumes you mean "this". I for one write it anyways, as it is just good coding practice. You can go back to it, and glide right over code and know whats happening because the "this" is highlighted.

[mbenney]

a simple way of checking circles against all other shapes is to put an invisible edge round your shapes, that is the radius of the circle) and test the mid point of the circle, giving the impression of seamless collision detection

nb only works if one shape is a circle

[Ed Mack]

Originally posted by Psytrix
Could you explain that code a lil better, im not sure exactly how it works...?

Have a look at the fla (one of my faves):

[mosterdfles_flash]

The only problem is that the movement in computer is not the same as in realLife...

In realLife when an object goes for about 5 meter per second and then hits a very strong wall... a hit has been done...

But with the computer, when an object moevs for 50 pixels and between that 'jump' is a small wall... theres no hit

could someone please fix this? (I.m trying to but with no succes :S

heres a small .fla with my problem (if you didn;t understand hat I ment ) (ITs MX!!!)

[Ed Mack]

For this there are two fixes:

Define the walls as sets of points (a bent line) and the balls movement as a vector. Then just test the vector against the line(s). This requires a good bit of algebra knowledge, but can be a better system in the long run for collisions (just find the gradient of the line) and mass hitTesting.

OR

Do a set of checks, ie. one for every 10px the ball will travel that frame.