c# Form: Check where two rectangles collided

Question

        foreach (var rect in listRect)
        {
            foreach (var rect2 in listRect)
            {
                if (Rectangle.Intersect(rect.r,rect2.r) != Rectangle.Empty && rect != rect2)
                {
                    if (rect.r.Left < rect2.r.Right) 
                        rect.speedX = ran.Next(-6,-4);
                        rect2.speedX = ran.Next(4, 6);

                    if (rect.r.Right > rect2.r.Left)
                        rect.speedX = ran.Next(4, 6);
                        rect2.speedX = ran.Next(-6, -4);

                    if (rect.r.Top < rect2.r.Bottom)
                        rect.speedY = ran.Next(4, 6);
                        rect2.speedY = ran.Next(-6, -4);

                    if (rect.r.Bottom > rect2.r.Top)
                        rect.speedY = ran.Next(-6, -4);
                        rect2.speedY = ran.Next(4, 6);

This is my current code for a function that is supposed to check if two rectangles have collided. Then if a collision is detected it is supposed to reverese the speed of the rectangles so they move in opposite directions.

My code as you probably can imagine does not work and i can't really figure out how I am supposed to solve this.

The rectangles are saved in a list called listRect and the class for the rectangles look like this:

class myRectangle
{
    public Rectangle r = new Rectangle();
    public int speedX = 5;
    public int speedY = 5;
}

How do i make simple collision detection so i know where they collided and how can i use that in my if-else statements?

are the rectangles moving along an axis? are they spinning or do they stay parralel to one another? — XikiryoX
– XikiryoX, Commented Oct 21, 2013 at 7:57
They are moving "freely" along X,Y axis and can collide from any direction with each other. No spinning or stuff like that. — PhilipSa
– PhilipSa, Commented Oct 21, 2013 at 7:59
I added a possible soluution below, I think rectangle.intersect might be a good way forward. Good Luck! — XikiryoX
– XikiryoX, Commented Oct 21, 2013 at 8:14

progpow · Accepted Answer · 2013-10-21 08:27:29Z

1

First you must compare first to other, by example

for(int first=0; first<listRect.Count;first++)
{
    for(int second=first+1;second<listRect.Count;second++)
    {
    }
}

Next step you must check position rectangles

if(listRect[first].r.X<listRect[second].r.X)
{
   if(listRect[first].r.Right > listRect[second].r.Left)
   {
       listRect[first].speedX = ran.Next(-6,-4);
       listRect[second].speedX = ran.Next(4, 6);
   }
}
else
{
    if(listRect[first].r.Left < listRect[second].r.Right)
    {
       listRect[first].speedX = ran.Next(4, 6);
       listRect[second].speedX = ran.Next(-6,-4);
    }
}

And similarly for top and down.

edited Oct 21, 2013 at 8:27

answered Oct 21, 2013 at 8:02

progpow

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6 Comments

PhilipSa Over a year ago

List<myRectangle> listRect = new List<myRectangle>(); This is how i iniaztilze my list I am not sure its compatible with your solution.

PhilipSa Over a year ago

Oh thanks! Do i define the centerX property in my rectangle class?

progpow Over a year ago

At which property of rectangle you add speedX?

progpow Over a year ago

Oh.. no.. How you define current position of Rectangle?

PhilipSa Over a year ago

Uh i don't but it would be using the rectangle.r.X and rectangle.r.Y as defiend in the c# rectangle code.

|

navit · Accepted Answer · 2013-10-21 08:19:18Z

I think you should not compare the Left of an element to the Right of another one. Consider this code:

                if (rect.r.Left < rect2.r.Left) 
                    rect.speedX = ran.Next(-6,-4);
                    rect2.speedX = ran.Next(4, 6);

                if (rect.r.Left > rect2.r.Left)
                    rect.speedX = ran.Next(4, 6);
                    rect2.speedX = ran.Next(-6, -4);

                if (rect.r.Top < rect2.r.Top)
                    rect.speedY = ran.Next(4, 6);
                    rect2.speedY = ran.Next(-6, -4);

                if (rect.r.Top > rect2.r.Top)
                    rect.speedY = ran.Next(-6, -4);
                    rect2.speedY = ran.Next(4, 6);

You just need to know which rectangle is on which side of each rectangle. Also I would suggest you to do this:

                    rect.speedX = -1 * rect.speedX;
                    rect2.speedY = -1 * rect.speedY;

Checking which rectangle is where relative to the other one doesn't show you the collision side.

XikiryoX · Accepted Answer · 2013-10-21 08:39:40Z

How about using rectangle.intersect? I think this has the functionality you want and saves you some coding..

You can use it this way:

if (Rect.r.IntersectsWith(rect2.r))
                {
                    if (rect.r.Left < rect2.r.Left) 
                        rect.speedX = ran.Next(-6,-4);
                        rect2.speedX = ran.Next(4, 6);

                    if (rect.r.Left > rect2.r.Left)
                        rect.speedX = ran.Next(4, 6);
                        rect2.speedX = ran.Next(-6, -4);

                    if (rect.r.Top < rect2.r.Top)
                        rect.speedY = ran.Next(4, 6);
                        rect2.speedY = ran.Next(-6, -4);

                    if (rect.r.Top > rect2.r.Top)
                        rect.speedY = ran.Next(-6, -4);
                        rect2.speedY = ran.Next(4, 6);
                }

You can find the documentation on msdn.

EDIT: you also just need to compare left to left and top to top (or right to right and bottom to bottom etc...) to change your speed, I edited the code snippet.

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