protected override void Update(GameTime gameTime)
{

    GamePadState PlayerOne = GamePad.GetState(PlayerIndex.One);

    // Move the camera using thumbsticks
    MoveCamera(PlayerOne);

    // Start or stop the animated sprite using buttons
    if (PlayerOne.Buttons.A == ButtonState.Pressed)
        explosion.Play();
    if (PlayerOne.Buttons.B == ButtonState.Pressed)
        explosion.Stop();

    // Update the animated sprite
    explosion.UpdateFrame((float)gameTime.ElapsedGameTime.TotalSeconds);

// Create a total bounding sphere for the mesh
BoundingSphere totalbounds = new BoundingSphere();
foreach (ModelMesh mesh in Ring.Meshes)
{
    totalbounds = BoundingSphere.CreateMerged(totalbounds, mesh.BoundingSphere);
}

// Project the center of the 3D object to the screen, and center the
// sprite there
Vector3 center = GraphicsDevice.Viewport.Project(totalbounds.Center,
    projectionMatrix, Camera1.ViewMatrix, Matrix.Identity);
explosionpos.X = center.X;
explosionpos.Y = center.Y;

    // Create a bounding box from the bounding sphere, and find the corner
    // that is farthest away from the center using Project
    BoundingBox extents = BoundingBox.CreateFromSphere(totalbounds);
    float maxdistance = 0;
    float distance;
    Vector3 screencorner;
    foreach (Vector3 corner in extents.GetCorners())
    {
        screencorner = GraphicsDevice.Viewport.Project(corner,
        projectionMatrix, Camera1.ViewMatrix, Matrix.Identity);
        distance = Vector3.Distance(screencorner, center);
        if (distance > maxdistance)
            maxdistance = distance;
    }

    // Scale the sprite using the two points (the sprite is 
    // 75 pixels square)
    explosion.Scale = maxdistance / 75; 
    
    base.Update(gameTime);
}

protected override void Draw(GameTime gameTime)
{
    GraphicsDevice.Clear(Color.CornflowerBlue);

    //Draw the model, a model can have multiple meshes, so loop
    foreach (ModelMesh mesh in Ring.Meshes)
    {
        //This is where the mesh orientation is set, as well as our camera and projection
        foreach (BasicEffect effect in mesh.Effects)
        {
            effect.EnableDefaultLighting();
            effect.World = Matrix.Identity * Matrix.CreateRotationY( RingRotation )
                * Matrix.CreateTranslation( RingPosition );
            effect.View = Camera1.ViewMatrix;
            effect.Projection = projectionMatrix;
        }
        //Draw the mesh, will use the effects set above.
        mesh.Draw();
    }

    // Draw the sprite over the 3D object
    spriteBatch.Begin( SpriteBlendMode.AlphaBlend, SpriteSortMode.Deferred, SaveStateMode.SaveState );
    explosion.DrawFrame( spriteBatch, explosionpos );
    spriteBatch.End();

    base.Draw( gameTime );
}