Platformer physics - hitTests + Gravity

[Sammo]

Hey, I figured this would be a popular subject, so i did search the forums, but didn't find anything relevent to my point. Whether I can search correctly, I'm sure i'll be corrected on

Anyway, I'm a novice at AS, but I want to create a platformer (don't worry! I'm not going to ask you lot to do it!), just for fun + practice. It'll be Mario-esque, just for the lack of creativity needed to create it.
I can handle all the controls and DuplicateMovieClip() etc. It's the hitTest + gravity i'm worried about. Here is an example of my hitTests:

onClipEvent (load) {
	speed = 5;
}
onClipEvent (enterFrame) {
	if (Key.isDown(Key.LEFT)) {
		_x -= speed;
	}
	if (this.hitTest(_root.wall_left)) {
		_x += speed;
	}
	if (Key.isDown(Key.RIGHT)) {
		_x += speed;
	}
	if (this.hitTest(_root.wall_right)) {
		_x -= speed;
	}
	if (Key.isDown(Key.UP)) {
		_y -= speed;
	}
	if (this.hitTest(_root.wall_up)) {
		_y += speed;
	}
	if (Key.isDown(Key.DOWN)) {
		_y += speed;
	}
	if (this.hitTest(_root.wall_down)) {
		_y -= speed;
	}
}
/*wall_up,down,left,right are instance names of a 
movieclip called wall which is on the stage 4 times. 
Boundries, wall_up at the top of the stage, etc.*/

Which is all very simple, but it works, so it's been fine for me.

When I introduce gravity (which I need help with, but will get onto in a moment), The character won't fall with an even pace, so i wont be able to counter the speed as I have done in the code above.

Is there a more technical way of using hitTests to stop a character?

And how would I implement gravity (I have searched and read the forums, and read Poms tutorial but im still not sure) into a game, such as a 2D platformer?

[JoMan]

I taught some people how to make a platform game a while back:

http://www.kirupaforum.com/forums/sh...ad.php?t=69442

It's outdated (cause I no longer use onClipEvents handlers), but it might help. Also, I use this ActionScript for the proper ground setting:

function collision() { 
    with (hero) { 
        if (limits.hitTest(_x-8, _y-30, true)) { 
            _x+=3; 
        } else if (limits.hitTest(_x+8, _y-30, true)) { 
            _x-=3; 
        } 
         
        if (limits.hitTest(_x-6, _y+5, true) and (jumpPower<=1)) { 
            _y-= jumpPower; 
        } 
    } 
}

I hope that I have helped you .

P.S. You should go to www.gotoAndPlay.it, and look at some of the hitTest articles that they have there.







peace

[Sammo]

Thanks JoMan I'm reading some articles now. But I still don't get the whole hitTest thing.

I mean, for all the hitTest examples I've seen they are countering the speed for eg:

if (Key.isDown (Key.UP)) {
      this._y -= 5;
 }
 if (hitTest(whatever_mc)) {
      this._y += 5;
 }

Right?
But when gravity is involved his falling speed is always raising. So how do you stop him when you can't be sure on his final impact speed?

[JoMan]

Oh, here is the ground ActionScript that I forgot to put with the other code:

for (i=1; i<=50; i++) {
if (!limits.hitTest(_x, _y-i, true) and (jumpPower<=1)) {
_y -= i-3;
break;
} else if (limits.hitTest(_x, _y-i, true) and (jumpPower<=1)) {
jumping = 0;
}
}

It will set the movieClip at the propper ground position once he makes contact with the limits. Also, the basic code:

if (limits.hitTest(_x, _y, true)) {
// resistance
}

can determine the x or y collision depending on the x or y property that you state, and it can also be determined by resistance, whether its x resistance, or y resistance. I hope I'm not confusing you; If so, tell me.





peace

[Sammo]

Sorry, can you explain what that code actually means?

[JoMan]

Ok, sure . First, what we have, is the:

for (i=1; i<=50; i++) {

The 'i' will become our resistance and propper ground setting. Now, when we add:

if (!limits.hitTest(_x, _y-i, true) and (jumpPower<=1)) {
    _y -= i-3;
    break;

This simply states that if the ground is not being touched (the '!' declares a negotive response), then the movieClip will slowly fall. The falling will not be a part of the gravity, but it is needed for the propper ground collision (instead of the movieClip landing above or in the ground ). Please reply for the next part.





peace

[Sammo]

Yup got that, is limits a variable you defined in some other place?

[Inferno]

cool new avatar joman

[JoMan]

Originally Posted by FlashNewbi3

cool new avatar joman

thanks .

[JoMan]

Originally Posted by Halikidiki

Yup got that, is limits a variable you defined in some other place?

No, 'limis' is the name of my terrain (ground) movieClip.

[Sammo]

Okay, thanks for this joMan.

Think you could go on with the code?

[JoMan]

Originally Posted by Halikidiki

Okay, thanks for this joMan.

Think you could go on with the code?

Sure, but it will have to wait until 10:00 tonight, because I have a fencing class... ENGARDE! Seeya then!






peace

[JoMan]

Ok, I'm back! Sorry for the delay. Ok, so now to declare the actual collision:

} else if (limits.hitTest(_x, _y-i, true) and (jumpPower<=1)) {
jumpPower = 0;
fallSpeed = 0;
}
}
}
}

Now, for my game, the jumpPower is the ammount of -well- jump power and the fallSpeed is the ammount of falling speed, so I declared -in this code- that both speeds will be set back to zero. Pretty basic stuff. Reply when you read this.






peace

[Sammo]

Yo, that's ok...I think...

So would I do:

onClipEvent (enterFrame) {
      if (Key.isDown (Key.UP)) {
           jumpPower = 10;
      }
 }

If yes, how and why?

If no, how would I do it?

[JoMan]

Well, the jumping code will go like this:

function mcJump() { // or an onClipEvent handler, depending on type of programming
if (Key.isDown(Key.UP) and (!jumping)) { // if the UP key is being pressed, and jumping is false
jumpPower = 15;
jumping = true;
}
if (jumping) { // if jumping is true
jumpPower--;
if (jumpPower>=15) { // if jumpPower is more than 15
jumpPower = -15;
}
mc._y -= jumpPower;
}
}

And in the ground collision code, just state that jumping is 'false', and jumpPower is set to '0'. Please reply when you read this.






peace