Elysian Shadows

Posted: **Thu Jan 05, 2012 12:35 pm**

I'm trying to implement the method of collision resolution described here and I can't get it to work.

Here's the code:


bool IsCollision(Rectangle rectangleA, Rectangle rectangleB)
{
	if (rectangleA.x < rectangleB.x + rectangleB.w &&
	    rectangleA.x + rectangleA.w > rectangleB.x &&
	    rectangleA.y < rectangleB.y + rectangleB.h &&
	    rectangleA.y + rectangleA.h > rectangleB.y)
	{
		return true;
	}
	return false;
}

...

if (IsCollision(rectangleA, rectangleB))
        {
            Rectangle overlap;
            overlap.h = rectangleB.y + rectangleA.h- rectangleB.x;
            overlap.w = rectangleA.x + rectangleA.w - rectangleA.y;
            Rectangle centerPointA;
            centerPointA.x = rectangleA.x + rectangleA.w / 2;
            centerPointA.y = rectangleA.y + rectangleA.h / 2;
            Rectangle centerPointB;
            centerPointB.x = rectangleB.x + rectangleB.w / 2;
            centerPointB.y = rectangleB.y + rectangleB.h / 2;
            if (overlap.w < overlap.h)
            {
                if (centerPointA.x < centerPointB.x)
                {
                    rectangleA.x -= overlap.w;
                }
                else
                {
                    rectangleA.x += overlap.w;
                }
            }
            else
            {
                if (centerPointA.y > centerPointB.y)
                {
                    rectangleA.y -= overlap.h;
                }
                else
                {
                    rectangleA.y += overlap.h;
                }
            }
        }

Posted: **Thu Jan 05, 2012 1:43 pm**

Oh dear...

First of all, you might want change your detection code slightly, before you get onto resolution:

Code: Select all

bool IsCollision(Rectangle rectangleA, Rectangle rectangleB)
{
   if (rectangleA.x < rectangleB.x + rectangleB.w && // Should be: Ax + Bw > Bx
       rectangleA.x + rectangleA.w > rectangleB.x && // Should be: Ax - Bw < Bx
       rectangleA.y < rectangleB.y + rectangleB.h &&  // Should be: Ay + Bh > By
       rectangleA.y + rectangleA.h > rectangleB.y)       // Should be: Ay - Bh < By
   {
      return true;
   }
   return false;
}

You're supposed to be adding and subtracting the alternate object's dimensions from the position of the object you're checking, thus if all statements are correct there must be a collision.

Like so:

Code: Select all

if ( A->x + B->w > B->x && A->x - B->w < B->x 
     && A->y + B->h > B->y && A->y - B->h < B->y )
{
    // Hooray for collision!
}

For the resolution:

Code: Select all

Rectangle overlap;

overlap.h = rectangleB.y + rectangleA.h - rectangleB.x;
overlap.w = rectangleA.x + rectangleA.w - rectangleA.y;

..Wtf?
To calculate how much A is overlapping B, you take Ax, add Aw, and then subtract Ay?

First calculate the distance between the objects, then I'm sure you'll find it fairly easy to get the overlap of the two

Think of the distance as a line between the two objects, the value of this line itself can be used to represent the overlap.

If the distance between the two on the X-axis is 40, and the width of both A and B are 44, they are overlapping each other by 4-units along this axis. Apply this to the Y-axis and poof, you've got your overlaps.

So for the distance between A and B you would do:
Distx = Ax - Bx
Disty = Ay - By

Code: Select all

Rectangle centerPointA;
centerPointA.x = rectangleA.x + rectangleA.w / 2;
centerPointA.y = rectangleA.y + rectangleA.h / 2;

Rectangle centerPointB;
centerPointB.x = rectangleB.x + rectangleB.w / 2;
centerPointB.y = rectangleB.y + rectangleB.h / 2;

This section is okay, but I would advise you to handle the objects from the center in the first place, so this wouldn't be necessary.

Have a read through this:
viewtopic.php?f=13&t=5767&p=69710&hilit ... ial#p69710

Let me know if you find any more problems.

Posted: **Thu Jan 05, 2012 2:54 pm**

Thanks for your help, I got it working by copying your code in the other tutorial. I understand how it's done now.

Elysian Shadows

Collision Resolution Help

Collision Resolution Help

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Re: Collision Resolution Help