Testing for Collisions

Demonstrates how to use the BoundingSphere class to check whether two models are colliding.

Overview

Collision detection is a crucial part of almost any game where the game needs to know whether or not two objects are touching. Normally, a simple distance (Vector3.Distance) check is sufficient, but when you need to know where they are touching or require more accurate information, that is when collision detection becomes critical.

Detecting Whether Two Models Collide

1. Track the position of a model as it moves about the game world.

   struct WorldObject
   {
       public Vector3 position;
       public Vector3 velocity;
       public Model model;
       public Texture2D texture2D;
       public Vector3 lastPosition;
       public void MoveForward()
       {
           lastPosition = position;
           position += velocity;
       }
       public void Backup()
       {
           position -= velocity;
       }
       public void ReverseVelocity()
       {
           velocity.X = -velocity.X;
       }
   }
   

2. Make a nested loop with the first model's meshes as the outer loop and the second model's meshes as the inner loop.

3. Inside the loop, follow these steps.

   1. Get the bounding sphere for the current mesh of the first model and the current mesh of the second model.

   2. Offset the bounding spheres by the current positions of the models.

   3. Call the BoundingSphere.Intersects method to check the pairs of bounding spheres for collision.

      If the method returns true, the objects are colliding.

   4. If the models are colliding, break out of the loop.

      static void CheckForCollisions(ref WorldObject c1, ref WorldObject c2)
      {
          for (int i = 0; i < c1.model.Meshes.Count; i++)
          {
              // Check whether the bounding boxes of the two cubes intersect.
              BoundingSphere c1BoundingSphere = c1.model.Meshes[i].BoundingSphere;
              c1BoundingSphere.Center += c1.position;
      
              for (int j = 0; j < c2.model.Meshes.Count; j++)
              {
                  BoundingSphere c2BoundingSphere = c2.model.Meshes[j].BoundingSphere;
                  c2BoundingSphere.Center += c2.position;
      
                  if (c1BoundingSphere.Intersects(c2BoundingSphere))
                  {
                      c2.ReverseVelocity();
                      c1.Backup();
                      c1.ReverseVelocity();
                      return;
                  }
              }
          }
      }
      

See Also

• Rotating and Moving the Camera

Concepts

• Bounding Volumes and Collisions

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