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Paddle Collision

Now it's time to make the ball bounce off of the paddle. You want to make sure that the ball hits only off the top part of the paddle and take into account the radius of the ball.

The following figure shows an illustration of the valid range:

The ball should collide against the paddle if the X position of the ball is within the valid X collision range and if the Y position of the ball is within the valid Y collision range.

Looking at the figure, you should be able to determine the following valid range (remembering that positive Y is down):

(ball.position.X > (paddle.position.X - radius - paddle.Width / 2)) &&
(ball.position.X < (paddle.position.X + radius + paddle.Width / 2)) &&
(ball.position.Y < paddle.position.Y) &&
(ball.position.Y > (paddle.position.Y - radius - paddle.Height / 2))

You want to do these paddle collision checks in CheckCollisions before you check for the wall collisions. If the ball position is within the valid range for the paddle collision, you want the ball to bounce.

For now, go ahead and just negate the Y component of the ball's direction, as you did for the wall. Test it and see whether the ball bounces off the paddle as you would expect it to.

If you run the game a few times, notice that the trajectory of the ball bouncing off the paddle is very predictable. This means that the player really has no agency over where the ball will go. It will always go to the same spots, provided that the player doesn't miss, which isn't very exciting.

Instead, you want to have some control over where the ball will end up going, depending on what part of the paddle it hits.

To solve this, you can't just negate the Y of the ball's direction. Instead, you need to employ vector reflection, which occurs when one vector is reflected off of another vector.

The following figure shows two different instances of the ball direction being reflected off the paddle:

Notice that the ball hitting off the left part of the paddle has a steeper reflected direction because the vector you're reflecting it about is steeper.

Luckily, MonoGame has a Vector2.Reflect function that takes in the original vector and the vector you're reflecting about, and returns the reflected vector.

So what you want to do is reflect off of a different vector, based on which third of the paddle is hit.

If the ball hits the left third, use (-0.196f, -0.981f). If the center third is hit, use (0, -1). And finally, if the right third is hit, use (0.196f, -0.981f).

For example, to reflect the ball off of the center of the paddle, the code is as follows:

ball.direction = Vector2.Reflect(ball.direction, new Vector2(0, -1));

Try to figure out the Boolean checks to determine which third of the paddle you hit, and reflect the ball.direction based on that.

After you implement this improved bouncing, the ball should bounce at different angles, depending on where you hit it. But you also will likely notice a major issue: Sometimes the ball goes straight through the edges of the paddle rather than just reflecting!

If the ball collides with the paddle on frame a, you reflect the direction of the ball. But if the ball is still colliding with the paddle on frame a + 1, the direction gets reflected again, which means it will either get stuck in the paddle or go straight through the paddle.

The ideal way to solve this is to use a more advanced form of collision detection (some of which I cover in Chapter 7 of my book). But because the game is relatively simple, you'll just use a hack: If the ball collides with the paddle, don't allow the ball and paddle to collide for the next 20 frames.

To implement this, create a new int member variable that is initially set to 0. As long as the value is 0, you should allow ball vs. paddle collisions in CheckCollisions. Then, when a ball and paddle collision occurs, set this int to 20. You then decrement that int once per frame until it gets back to zero.

This solution is imperfect—you will notice that there are still some cases to get the ball/paddle collision to act a bit weird.

You'll fix most of the situations in Part 4 of this series, but ultimately the only way to make it perfect is to use more accurate collision detection.


Now add some blocks. First, make a new class called Block that inherits from GameObject. For now, set the textureName to "block_red" in the constructor and don't have any other code in Block.

In Game1, you'll create a list of all the active blocks. You can declare a list with the following code:

List<Block> blocks = new List<Block>();

Once you have this list, you need to create some blocks to put in it. Later in this series, you'll actually load the block locations from a file. For now, though, you'll create a row of 15 blocks.

In LoadContent, create a for loop that loops 15 times. It should create the first Block at (64, 200) and the last Block at (960, 200).

Every time you create a new Block, you need to add it to the blocks list with blocks.Add. The loop should look something like this:

for (int i = 0; i < 15; i++)
   Block tempBlock = new Block(this);
   tempBlock.position = new Vector2(64 + i * 64, 200);

Once you create this list of blocks, it's time to draw them. In Game1.Draw, after ball.Draw, but before spriteBatch.End, add the following foreach loop:

foreach (Block b in blocks)

If you run the code now, you should see a row of red blocks, as shown in the following figure:

Now that you have the blocks drawing, you can add collision detection. It will be very similar to what you did for the paddle, except that in the case of a block, you want the ball to be able to collide with any of the four sides.

If the ball hits on the left or right side, you should negate the X direction; conversely, if it is at the top or bottom, you should negate the Y direction.

For now, you will just remove a block if it gets hit, so you don't need a frame counter as you used for the paddle.

I suggest drawing a diagram similar to the one for the paddle collision. Once you've figured out the collision ranges, it should be relatively straightforward to figure out which comparison checks you should make.

You want to loop through each block and check to see whether a particular block collides with the ball. Because the ball can hit only one block at a time, you can break out of the loop when you detect the first collision.

One final issue is removing the block from the list. There's a Remove function you can call on the list that takes in the object you want to remove. The only problem, however, is you can't modify a container while iterating through it with foreach.

One way to solve this problem is to save off the Block you collide with into a temporary variable (that exists outside the scope of the foreach) and then remove that block (if it exists) after the loop is over.

It might take a little while to figure out the conditionals for the collision. After you do, you'll end up with a row of blocks that you can break.

In any event, that's it for this article. In Part 3 of this series, you'll add different color blocks, sound effects, and some fun power-ups that add variability to the game.

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