Collision Detection Between Rectangles - Resulting Angles

Question

Basically I have a bunch of rectangles floating around at 8 different angles (45 degrees, 90 degrees etc). I have collision detection going on between all of them, but one thing still doesn't work as it should. I don't know if I'm just not thinking or what, but I can't seem to get the resulting angles right. I've also tried searching multiple places, but haven't really gained anything from what I've found.

NOTE: the angle system here starts at 0 at the top and increases clockwise.
NOTE: all rectangles have the same mass

Say one rectangle going straight right (90 degrees) hits another going straight left (270 degrees). They will hit off of each other just fine.

Now say one going left gets hit by one going up. Here I can't simply reverse the angles or anything.

If you have more than one way, consider the following: unless I rearrange the CD so it spreads into the other code, I have the positions of all of the rectangles. The CD checks by seeing if two are overlapping, not by comparing where they are going.

As I've been working on pretty much everything except for the collision detection until now, I only have tonight left to get it working and add a few other small things before I'm done.

If you know of a way to make the angles come out right without hardcoding, great. At this point I'm ready to hardcode it (not too much really) if all I have is which rectangle hits the other (ex 2), or if they both do (ex 1). Either one is really helpful.

The resulting angle depends not only on how they were moving but also on the contact point. Are you making some assumption about the contact point? — Vaughn Cato
I have it figuring out if it hit the side or the top/bottom if that's what you mean. To be honest, thinking about it now, the CD between the rectangles could actually afford to go if need be. It won't make too much of a difference, I just like things a certain way >.> — chris
what do you mean by 8 different angles? are the rectangles rotated around an axis or straight? Do you mean direction rather than angle? — SD1990
Sorry about the wording, I did mean directions. They travel N,NE,E,SE,S,SW,W,NW. The thing is that whenever I think about what the new directions would be, I at least need to know which one hit which. I've looked at elastic collision things, but they're all simpler in the way that all examples I can find are circles. I did finish this code and just left this out since it wasn't important, but a simple way to do rectangular collision while they're travelling in any two given directions would be nice for future endeavours. — chris

SD1990 SD1990 · Accepted Answer · 2011-10-05T14:09:56

I think you mean something like this...

Each Rectangle has this functionality, testing against, say an array of other objects.

Obstacle* obstacle = new Obstacle;
Obstacle** obstacles = obstacle*[];

For(int i = 0; i <3; i++)
{
obstacles[0] = New Obstacle(x,y, etc...);
etc...
}

Or something similar... this is a little rusty

 void collision(obstacles)
    {
        for(int i = 0; i < obstacles.sizeOf();i++)
           {
            //bottom y
            if((this.ypos + this.height) > (obstacles[i].ypos - obstacles[i].height))
                obstacles[i].doYCollision(this);


            //top y
            if((this.ypos - this.height) < (obstacles[i].ypos + obstacles[i].height))
                obstacles[i].doYCollision(this);


            //right x
            if((this.xpos + this.width) > (obstacles[i].xpos - obstacles[i].width))
                obstacles[i].doXCollision(this);

            //left x
            if((this.xpos - this.width) < (obstacles[i].xpos + obstacles[i].width))
                obstacles[i].doXCollision(this);
           }
    }

again im a little rusty but if you follow it you should be able to relaise what im doing.

then all you need is the resulting function calls.

void doYCollision(Obstacle obstacle)
{
  // Whatever y direction they are going do the opposite
  obstacle.yDir = obstacle.yDir * -1;
}
void doXCollision(Obstacle obstacle)
{
  // Whatever x direction they are going do the opposite
  obstacle.xDir = obstacle.xDir * -1;
}

where yDir, xDir is the x and y velocity of the current object.

i should point out again this is very rusty and without having some code from you this is the best ive got. but either way this should start you off into collision detection, the code above shoudl allow for all collisions with all obstacles/objects/pink flamingos/ whatever youve got. Im hoping also that itll do what you want when it comes to multiple collisions at the same time.

You shouldnt need to worry too much about the exact angle (unless you need it for something else), as velocity is a vector mass so has both speed and direction and you can get direction by treating x and y as two different elements. You can do this using the dot product method aswell => http://www.topcoder.com/tc?d1=tutorials&d2=geometry1&module=Static, but if they are just rectangles this should be fine.

Hopes this helps

Collision Detection Between Rectangles - Resulting Angles

1 Answers