Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

238 Chapter 10

(picx, picy)

(paddlex, paddley)

(paddlex, paddley)

paddlew

(picw/2) (picw/2)

(picx, picy)

Figure 10-4: Two collision cases between the paddle and our smiley ball

Because we want the ball to bounce realistically off the paddle,

we want to check for the cases where the bottom center of the ball

just touches the corners of the paddle at the left and right extremes.

We want to make sure the player scores a point not only when the

ball bounces directly off the top of the paddle but also whenever it

bounces off the paddle’s corners. To do this, we’ll see if the ball’s

vertical location is near the bottom of the screen where the paddle

is, and if so, we’ll check whether the ball’s horizontal location would

allow it to hit the paddle.

First, let’s figure out what range of x-coordinate values would

allow the ball to hit the paddle. Since the middle of the ball would

be half the width of the ball across from its (picx, picy) top-left

corner, we’ll add the width of the ball as a variable in the setup

section of our app:

picw = 100

As shown in Figure 10-4, the ball could hit the top-left corner

of the paddle when picx plus half the width of the picture (picw/2)

touches paddlex, the x-coordinate of the left corner of the paddle.

In code, we could test this condition as part of an if statement: picx

+ picw/2 >= paddlex.

We use the greater than or equal to condition because the ball

can be farther right (greater than paddlex in the x direction) and

still hit the paddle; the corner case is just the first pixel for which

the player gets a point for hitting the paddle. All the x-coordinate

values between the left corner and the right corner of the paddle

are valid hits, so they should award the user a point and bounce

the ball back.

To find that top-right corner case, we can see from the figure

that we’re requiring the middle of the ball, whose x-coordinate