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bratiefanut

Bounding box collision in a platformer game

11 posts in this topic

Hello devs,

 

I am making a prototype for my first game (when I say "first" I mean that I camed up with the ideea myself, not games like pong or tic-tac-toe, which by the way, you can see here and here).

I had some problems with gravity but fixed it after a while. Next problem that I faced was bounding box collision detection... 

 

[attachment=19567:Capture.PNG]

With red are drawn the bounding boxes.

 

I can stay on the top the ground. But like in any other platformer, you will hit the platform from down, left and right too.

I don't really know how to code collision when player hit the platform from other sides.

Here is the code to stay above the platform. 

if(player.bbox.Intersect(&platform1.bbox))
{
      player.y = platform1.y1 - player.width; //platform1.y1 - top-left corner Y coordinate
}

My bounding box class give me acces just to: x1, y1, x2, y2 coordinates of the bounding box, Intersect function (that checks if 2 bounding boxes intersects) and TestPoint function(that checks if a point is inside a bounding box).

How I can do the collision with the other sides of the platform not only the top of it? 

Edited by Bratie Fanut
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The logic is just the same, you could just swap X and Y for that matter...
 
Using the same approach you used on your own function:
if(player.bbox.Intersect(&platform1.bbox))
{
	//Y axis
	if (player.y1 <= platform1.y1) //assuming y1 to be the "bigger-y coordinate"
		player.y1 = platform1.y1 - player.height; //To place it on top of the platform.
	else
		player.y1 = platform1.y2 + 1; //To place it under the platform when hit from below.
	//X axis
	if (player.x1 >= platform1.x1) //assuming x1 to be the "smaller-x coordinate"
		player.x1 = platform1.x2 + 1;
	else
		player.x1 = platform1.x1 - player.width; //To move it aside from the intersected platform.
}
This could probably work on general situations, with moderate speeds and all that.
I am assuming that (y1, x1) is the top-left coordinate, and (x2, y2) is the bottom-right one.
You'll probably have to adjust the code.
 
I guess I've been using Box2D for too long, getting lazy. Edited by dejaime
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The logic is just the same, you could just swap X and Y for that matter...

 

Using the same approach you used on your own function:

if(player.bbox.Intersect(&platform1.bbox))
{
	//Y axis
	if (player.y1 >= platform1.y1) //assuming y1 to be the "bigger-y coordinate"
		player.y1 = platform1.y1 - player.height; //To place it on top of the platform.
	else
		player.y1 = platform1.y2 + 1; //To place it under the platform when hit from below.
	//X axis
	if (player.x2 >= platform1.x1) //assuming x1 to be the "smaller-x coordinate"
		player.x1 = platform1.x1 - player.width; //To move it aside from the intersected platform.
	else
		player.x1 = platform1.x2 + 1;
}

This could probably work on general situations, with moderate speeds and all that.

I am assuming that (y1, x1) is the top-left coordinate, and (x2, y2) is the bottom-right one.

You'll probably have to adjust the code.

 

Remember to reset the speeds and all that when it hit an immovable object!

 

I guess I've been using Box2D for too long, getting lazy.

You are assuming everything right. I am getting lazy too... I should think of another checking in that if statement..

I will try it to see if it works.

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I cover the topic in detail in this tutorial

 

Here is the code to detect a collision from any direction:

return (this.x <= rect.x + rect.width &&
                rect.x <= this.x + this.width &&
                this.y <= rect.y + rect.height &&
                rect.y <= this.y + this.height);

The explanation of why it works is in the tutorial.  I actually did a whole series of gamdev math tutorials that you may find useful.

 

Let me know if you have any questions.

I will check your tutorials. Thank you:)

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You are assuming everything right. I am getting lazy too... I should think of another checking in that if statement..
I will try it to see if it works.

My last code is wrong, the horizontal part, I have just fixed it, please take another look. Edited by dejaime
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Its not working.. the condition for y axis and x axis are true at the same time.. so when I jump on the platform I stay on top of it but because x axis conditions are true I am stucked.

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Yes, it needs another test level... Be back in a jiffy.
 
 
if(player.bbox.Intersect(&platform1.bbox))
{
	//Y axis
	if (player.y1 <= platform1.y1) {//assuming y1 to be the "bigger-y coordinate"
		player.y1 = platform1.y1 - player.height; //To place it on top of the platform.
	}
	else if (player.y1 >= platform1.y2) {
		player.y1 = platform1.y2 + 1; //To place it under the platform when hit from below.
	}
	//X axis
	else {
		if (player.x1 >= platform1.x1) //assuming x1 to be the "smaller-x coordinate"
			player.x1 = platform1.x2 + 1;
		else
			player.x1 = platform1.x1 - player.width; //To move it aside from the intersected platform.
	}
}
 
 
This should prevent it from updating both at the same time... It could also be something like this:

if(player.bbox.Intersect(&platform1.bbox))
{
	//Y axis
	if (/*vertical collision*/) {
		if (player.y1 <= platform1.y1) {//assuming y1 to be the "bigger-y coordinate"
			player.y1 = platform1.y1 - player.height; //To place it on top of the platform.
		}
		else {
			player.y1 = platform1.y2 + 1; //To place it under the platform when hit from below.
		}
	//X axis
	if (/*horizontal collision*/) {
		if (player.x1 >= platform1.x1) //assuming x1 to be the "smaller-x coordinate"
			player.x1 = platform1.x2 + 1;
		else
			player.x1 = platform1.x1 - player.width; //To move it aside from the intersected platform.
	}
}
Sorry for anything wrong, I'm just not used to code without obsessively testing every added code block. I guess I am not used to think ahead...
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