PZ-33 · Mini-Game — Asteroids Lite — Part 1 (Movement)

Mini-Game — Asteroids Lite — Part 1 (Movement)

The classic Asteroids ship rotates freely and drifts through space — wrap around one edge and you reappear on the opposite side. In Part 1 you build exactly that: rotation with actor.angle, thrust in the facing direction using a bit of trigonometry, and four-edge screen wrapping. No game states yet — that's Part 2.

⏱ 1 hour🎮 Game lesson (part 1 of 2)📚 After PZ-32💻 VS Code or Thonny

Learning Goals

3 min

By the end of this lesson you can:

Rotate an Actor by changing actor.angle in response to left/right key presses.
Convert the ship's angle to an (vx, vy) velocity using math.cos and math.sin, so thrust always points forward.
Wrap the ship around all four screen edges so it never disappears off-screen.

Warm-Up · Angles and the Unit Circle

5 min

In Pygame Zero, actor.angle is in degrees. The standard maths functions use radians. What does this print?

import math
angle_deg = 90
angle_rad = math.radians(angle_deg)
print(round(math.cos(angle_rad), 2))
print(round(math.sin(angle_rad), 2))

Show the answer

Output

0.0
1.0

At 90 ° the cosine is 0 and the sine is 1. That means "pointing right" in maths corresponds to a purely vertical thrust in screen space — we need to account for Pygame Zero's coordinate system where y grows downward. We will handle that in the concept section.

New Concept · Rotate, Thrust, Wrap

12 min

Think of the ship like a compass needle. Turning the needle does not move the compass — you still have to step forward. In code: changing actor.angle rotates the sprite image, but the ship only moves when you add to its velocity.

Rotating the actor

Pygame Zero's actor.angle starts at 0 (pointing right). Increasing the angle rotates counter-clockwise. To turn left, add to the angle; to turn right, subtract:

TURN_SPEED = 4

def update():
    if keyboard.left:
        ship.angle += TURN_SPEED
    if keyboard.right:
        ship.angle -= TURN_SPEED

Thrust in the facing direction

To move the ship forward we decompose the angle into vx and vy components. Because y grows downward on screen, we negate the sine term:

import math

THRUST = 0.3

def update():
    global vx, vy
    if keyboard.up:
        rad = math.radians(ship.angle)
        vx += math.cos(rad) * THRUST
        vy -= math.sin(rad) * THRUST
    ship.x += vx
    ship.y += vy

Screen wrapping

When the ship drifts past one edge it should reappear on the opposite side:

def wrap(actor):
    if actor.x > WIDTH:
        actor.x = 0
    if actor.x < 0:
        actor.x = WIDTH
    if actor.y > HEIGHT:
        actor.y = 0
    if actor.y < 0:
        actor.y = HEIGHT

Worked Example · The Full Movement Sketch

12 min

Wei Jie wants to recreate the Asteroids ship feel. He uses a plain white triangle as the ship image (save a white triangle PNG as images/ship.png, or use any small Actor image you have). Save as asteroids1.py:

# asteroids1.py — rotation, thrust, and screen wrap (Part 1)

import pgzrun
import math

WIDTH = 600
HEIGHT = 600
TITLE = "Asteroids Lite — Part 1"
TURN_SPEED = 4
THRUST = 0.25
DRAG = 0.99

ship = Actor("ship", center=(300, 300))
vx = 0.0
vy = 0.0

def wrap(actor):
    if actor.x > WIDTH:
        actor.x = 0
    elif actor.x < 0:
        actor.x = WIDTH
    if actor.y > HEIGHT:
        actor.y = 0
    elif actor.y < 0:
        actor.y = HEIGHT

def update():
    global vx, vy
    if keyboard.left:
        ship.angle += TURN_SPEED
    if keyboard.right:
        ship.angle -= TURN_SPEED
    if keyboard.up:
        rad = math.radians(ship.angle)
        vx += math.cos(rad) * THRUST
        vy -= math.sin(rad) * THRUST
    # apply drag so the ship doesn't accelerate forever
    vx *= DRAG
    vy *= DRAG
    ship.x += vx
    ship.y += vy
    wrap(ship)

def draw():
    screen.fill("black")
    ship.draw()
    screen.draw.text(
        "LEFT/RIGHT rotate  UP thrust",
        topleft=(10, 10),
        fontsize=18,
        color="grey",
    )

pgzrun.go()

A black 600×600 window with the ship sprite near the centre. Controls are shown top-left.

The DRAG = 0.99 multiplier shrinks velocity by 1 % each frame, giving the floating-in-space feel without infinite drift.

Try It Yourself

13 min

01 🟢 Speed limits

Add a constant MAX_SPEED = 6. After updating vx and vy, cap the speed so the ship never goes faster than MAX_SPEED. Hint: use math.hypot(vx, vy) to get the current speed.

MAX_SPEED = 6

speed = math.hypot(vx, vy)
if speed > MAX_SPEED:
    scale = MAX_SPEED / speed
    vx *= scale
    vy *= scale

02 🟡 Thruster flame effect

When the player holds UP, draw a small orange filled circle just behind the ship to simulate exhaust. Calculate the flame position using the opposite direction to thrust.

if keyboard.up:
    rad = math.radians(ship.angle)
    fx = ship.x - math.cos(rad) * 20
    fy = ship.y + math.sin(rad) * 20
    screen.draw.filled_circle((int(fx), int(fy)), 5, "orange")

Mini-Challenge — Add Static Asteroids

8 min

Combine today's ship movement with the random module from Level 1. Spawn 5 grey circles at random positions on the screen. They don't move yet — that's Part 2's job. Apply the same wrap logic to each circle so they stay on screen.

It works if…

5 grey circles appear at random positions,
the ship flies around them without crashing the program

Show one possible solution

import random

NUM_ASTEROIDS = 5
asteroids = [
    {"x": random.randint(0, WIDTH), "y": random.randint(0, HEIGHT)}
    for _ in range(NUM_ASTEROIDS)
]

def draw():
    screen.fill("black")
    for a in asteroids:
        screen.draw.filled_circle((int(a["x"]), int(a["y"])), 24, "grey")
    ship.draw()

Placing asteroids in a list of dicts sets you up perfectly for adding movement in Part 2.

Recap

3 min

You now have a ship that rotates with actor.angle, thrusts in the facing direction using math.cos/math.sin, and wraps around all four screen edges. A small drag multiplier keeps the physics feeling like outer space. Part 2 adds moving asteroids, bullets, and game states.

Vocabulary Card

actor.angle: The rotation of the sprite in degrees. 0 = facing right; increases counter-clockwise.
math.radians(deg): Converts degrees to radians so math.cos/math.sin work correctly.
screen wrap: When an object exits one edge it reappears on the opposite edge — keeps objects always on screen.
drag: Multiplying velocity by a value slightly below 1 (e.g. 0.99) each frame slows the object gradually, simulating friction or space resistance.

Homework

4 min

Add a brake key to the Part-1 ship. When the player holds DOWN, multiply both vx and vy by 0.92 each frame (stronger drag) so the ship slows down quickly. Save as asteroids1_brake.py and bring the file to class.

Sample · `asteroids1_brake.py`

# asteroids1_brake.py — adds a brake key to Part 1

import pgzrun
import math

WIDTH = 600
HEIGHT = 600
TURN_SPEED = 4
THRUST = 0.25
DRAG = 0.99
BRAKE = 0.92

ship = Actor("ship", center=(300, 300))
vx = 0.0
vy = 0.0

def wrap(actor):
    if actor.x > WIDTH:
        actor.x = 0
    elif actor.x < 0:
        actor.x = WIDTH
    if actor.y > HEIGHT:
        actor.y = 0
    elif actor.y < 0:
        actor.y = HEIGHT

def update():
    global vx, vy
    if keyboard.left:
        ship.angle += TURN_SPEED
    if keyboard.right:
        ship.angle -= TURN_SPEED
    if keyboard.up:
        rad = math.radians(ship.angle)
        vx += math.cos(rad) * THRUST
        vy -= math.sin(rad) * THRUST
    if keyboard.down:
        vx *= BRAKE
        vy *= BRAKE
    vx *= DRAG
    vy *= DRAG
    ship.x += vx
    ship.y += vy
    wrap(ship)

def draw():
    screen.fill("black")
    ship.draw()
    screen.draw.text("DOWN to brake", topleft=(10, 10), fontsize=18, color="grey")

pgzrun.go()

The brake just applies a stronger drag multiplier on the DOWN frame — same maths as DRAG, just a smaller value.

Mini-Game — Asteroids Lite — Part 1 (Movement)

⏱ 1 hour🎮 Game lesson (part 1 of 2)📚 After PZ-32💻 VS Code or Thonny

import math THRUST = 0.3 def update(): global vx, vy if keyboard.up: rad = math.radians(ship.angle) vx += math.cos(rad) * THRUST vy -= math.sin(rad) * THRUST ship.x += vx ship.y += vy

ship = Actor("ship", center=(300, 300)) vx = 0.0 vy = 0.0 def wrap(actor): if actor.x > WIDTH: actor.x = 0 elif actor.x < 0: actor.x = WIDTH if actor.y > HEIGHT: actor.y = 0 elif actor.y < 0: actor.y = HEIGHT

def update(): global vx, vy if keyboard.left: ship.angle += TURN_SPEED if keyboard.right: ship.angle -= TURN_SPEED if keyboard.up: rad = math.radians(ship.angle) vx += math.cos(rad) * THRUST vy -= math.sin(rad) * THRUST # apply drag so the ship doesn't accelerate forever vx *= DRAG vy *= DRAG ship.x += vx ship.y += vy wrap(ship) def draw(): screen.fill("black") ship.draw() screen.draw.text( "LEFT/RIGHT rotate UP thrust", topleft=(10, 10), fontsize=18, color="grey", ) pgzrun.go()

import random NUM_ASTEROIDS = 5 asteroids = [ {"x": random.randint(0, WIDTH), "y": random.randint(0, HEIGHT)} for _ in range(NUM_ASTEROIDS) ] def draw(): screen.fill("black") for a in asteroids: screen.draw.filled_circle((int(a["x"]), int(a["y"])), 24, "grey") ship.draw()

Learning Goals

Warm-Up · Angles and the Unit Circle

New Concept · Rotate, Thrust, Wrap

Rotating the actor

Thrust in the facing direction

Screen wrapping

Worked Example · The Full Movement Sketch

Try It Yourself

Mini-Challenge — Add Static Asteroids

Recap

Vocabulary Card

Homework

Sample · asteroids1_brake.py

Learning Goals

Warm-Up · Angles and the Unit Circle

New Concept · Rotate, Thrust, Wrap

Rotating the actor

Thrust in the facing direction

Screen wrapping

Worked Example · The Full Movement Sketch

Try It Yourself

Mini-Challenge — Add Static Asteroids

Recap

Vocabulary Card

Homework

Sample · asteroids1_brake.py

Sample · `asteroids1_brake.py`

Sample · `asteroids1_brake.py`