How to check for cube collisions?

Question

I want a method, which takes two "ObjectBox" objects (A "ObjectBox" has .getX() .getY() .getZ() .getSizeX() .getSizeY() .getSizeZ() methods) as a parameter and return true if the two Boxes are colliding and false if they aren't.

So it should be something like this:

public static boolean checkCollision(ObjectBox box1, ObjectBox box2){
return //TRUE IF COLLIDING - FALSE IF NOT COLLIDING
}

I tried figuring it out but it seemed pretty hard to me.

Answer 1

This is called an AABB (Axis Aligned Bounding Box). The collision checks for these are pretty fast and simple. Basically you just check their relative positions and see if all three axes overlap:

public static boolean checkCollision(ObjectBox a, ObjectBox b){
{
   //check the X axis
   if(Math.abs(a.getX() - b.getX()) < a.getSizeX() + b.getSizeX())
   {
      //check the Y axis
      if(Math.abs(a.getY() - b.getY()) < a.getSizeY() + b.getSizeY())
      {
          //check the Z axis
          if(Math.abs(a.getZ() - b.getZ()) < a.getSizeZ() + b.getSizeZ())
          {
             return true;
          }
      }
   }

   return false;
} 

Only if all three axes overlap there is a collision. For example, if the cubes were overlapping on the X and Z axes and not the Y, it means that the cubes are above or below each other and not colliding. You can optimize the code above slightly, if there's an axis you're more likely to be colliding on, put that check first so you don't need to check the others.

Keep in mind, the above code assumes that the position is the center of the cube and the size is getting the half extents of the cube. If your getSize_ functions are returning the full dimension, you'll need to half them for the above to work properly.

Answer 2

You could also consider doing the checks as a method in one of the objects. It might end up making your program less complex. For example instead of

public static boolean checkCollision(ObjectBox box1, ObjectBox box2){
    return //TRUE IF COLLIDING - FALSE IF NOT COLLIDING
}

You would have something like

public boolean checkCollision(ObjectBox other){
    return //TRUE IF COLLIDING - FALSE IF NOT COLLIDING
}

Which would be a public method inside of your ObjectBox class. Then you just have to do some basic Math inside of the box assuming that the boxes are 2D ( a little more Math if they are 3D) and invoke the new method as follows:

ObjectBox obj1 = new ObjectBox(/*Assume some parameters*/);
ObjectBox obj2 = new ObjectBox(/*Assume some parameters*/);
//Now we can compare them without needing the static method.
if(obj1.checkCollision(obj2))
    //Do amazing things.

Edit: Byte56 has you covered on the collision checking.