Add starfield example

examples/basic/stars.py

@@ -0,0 +1,91 @@
+import math
+import random
+from collections import deque
+
+WIDTH = 1000
+HEIGHT = 1000 * 9 // 16
+ACCEL = 1.0  # Warp factor per second
+DRAG = 0.71  # Fraction of speed per second
+MIN_WARP_FACTOR = 0.1
+BOUNDS = Rect(0, 0, WIDTH, HEIGHT)
+
+
+warp_factor = MIN_WARP_FACTOR
+centerx = WIDTH // 2
+centery = HEIGHT // 2
+stars = []
+
+
+class Star:
+    def __init__(self, pos, vel):
+        self.pos = pos
+        self.vel = vel
+        self.brightness = 10
+        self.speed = math.hypot(*vel)
+        self.prev_pos = deque(maxlen=3)
+        self.prev_pos.append(self.pos)
+
+    def set_pos(self, pos):
+        self.pos = pos
+        self.prev_pos.append(pos)
+
+
+def draw():
+    screen.clear()
+    for star in stars:
+        b = star.brightness
+        color = (b, b, b)  # a grey
+        screen.draw.line(star.prev_pos[0], star.pos, color)
+
+    screen.draw.text(
+        "Warp {:0.1f}".format(warp_factor),
+        fontsize=40,
+        midbottom=(WIDTH // 2, HEIGHT - 40),
+        color=(180, 160, 0),
+    )
+    screen.draw.text(
+        "Hold SPACE to accelerate",
+        fontsize=30,
+        midbottom=(WIDTH // 2, HEIGHT - 8),
+        color=(90, 80, 0),
+    )
+
+
+def update(dt):
+    global stars, warp_factor
+    if keyboard.space:
+        warp_factor += ACCEL * dt
+    warp_factor = (
+        MIN_WARP_FACTOR +
+        (warp_factor - MIN_WARP_FACTOR) * DRAG ** dt
+    )
+
+    for _ in range(int(dt * 200)):
+        angle = random.uniform(-math.pi, math.pi)
+        dx = math.cos(angle)
+        dy = math.sin(angle)
+        speed = 255 * random.uniform(0.3, 1.0) ** 2
+
+        d = random.uniform(0, 100)
+        pos = centerx + dx * d, centery + dy * d
+
+        v = speed * dx, speed * dy
+        stars.append(Star(pos, v))
+
+    for s in stars:
+        x, y = s.pos
+        vx, vy = s.vel
+
+        x += vx * warp_factor * dt
+        y += vy * warp_factor * dt
+        s.set_pos((x, y))
+        s.brightness = min(s.brightness + warp_factor * 200 * dt, s.speed)
+        s.vel = vx * 2 ** dt, vy * 2 ** dt
+    stars = [star for star in stars if BOUNDS.collidepoint(star.prev_pos[0])]
+
+
+# Jump-start the star field
+for _ in range(10):
+    update(0.5)
+for _ in range(5):
+    update(1 / 60)