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Touching and collisions: when two pieces meet

Catching, bouncing, crashing, collecting. Almost everything exciting in a game happens when two pieces meet. So your game needs a way to notice.

The big idea

a.touches(b) is True on every frame that two Things overlap. Programmers call this collision detection.

See it in code

1Start simple

touches() asks a yes-or-no question about two Things. Put it in an if and the game reacts the moment they overlap:

python
basket = Thing(x=320, y=420, width=100, height=24, color="teal")
cupcake = Thing(x=320, y=240, size=40, color="orchid", shape="circle")

def play():
    if not cupcake.touches(basket):
        cupcake.y = cupcake.y + 3

def draw():
    paper("misty rose")
    rect(x=320, y=455, width=640, height=50, color="burlywood")
    basket.draw()
    cupcake.draw()
    if cupcake.touches(basket):
        write("Touching!", x=320, y=60, size=48, color="sienna")
The cupcake has fallen onto the basket:
A bakery scene with a purple cupcake overlapping a teal basket, and the word "Touching!" written at the top.

The cupcake fell until touches turned True, then stopped. The check is made fresh on every frame, about 60 times a second.

2A step further

A touch lasts many frames. Add a point on every one and a single catch scores far too much. So a catch must end the touch, by sending the cupcake back to the top:

python
basket = Thing(x=320, y=420, width=100, height=24, color="teal")
start_y = -20
cupcake = Thing(x=320, y=start_y, size=40, color="orchid", shape="circle")
game.score = 0

def play():
    cupcake.y = cupcake.y + 5
    if cupcake.touches(basket):
        game.score = game.score + 1
        cupcake.y = start_y

def draw():
    paper("misty rose")
    rect(x=320, y=455, width=640, height=50, color="burlywood")
    basket.draw()
    cupcake.draw()
    write(f"Score: {game.score}", x=320, y=30, color="sienna")
After two catches, the score is exactly 2:
A bakery scene with a teal basket, a purple cupcake falling from the top again, and the score reading 2.

The moment they touch, the cupcake jumps back to start_y. On the next frame they no longer overlap, so the point is added just once.

3In the arcade

With a list of pieces, check each one in a loop. When the ball touches a brick, the brick is taken out of the list and the ball bounces back:

python
bricks = []
for col in range(8):
    bricks.append(Thing(x=50 + col * 77, y=80, width=70, height=22, color="tomato"))
ball = Thing(x=285, y=300, size=16, color="white", shape="circle", dx=0, dy=-6)

def play():
    ball.move()
    for brick in list(bricks):
        if ball.touches(brick):
            bricks.remove(brick)
            ball.dy = -ball.dy
            break

def draw():
    paper("black")
    for brick in bricks:
        brick.draw()
    ball.draw()
    write(f"Bricks left: {len(bricks)}", x=320, y=440, color="white")
The ball has knocked out a brick and bounced away:
A black game window with a row of eight red bricks with one missing where a white ball has hit it and bounced back down.

bricks.remove(brick) takes the brick out of the game for good. break stops the loop, so one hit can only ever remove one brick.

The same idea, everywhere

Change what touches what and you get a new game. A snake's head touching food. A ball touching a paddle. A hopper touching a pillar.

Try it yourself

In the catch game, take out the line cupcake.y = start_y and run it. Watch the score. Why does one catch now score so many points?

The common mistake

Asking a Thing to touch something that isn't a Thing. ball.touches(bricks) gives it the whole list, and the game says so. Loop over the list and check one brick at a time.

What it unlocks

Touching is a yes-or-no question, so it lives in an if. What happens next is usually score and lives, or a piece leaving a list.

Related concepts