Flow Field

1// Nature of Code - Flow Field
2// Inspired by Chapter 6: Autonomous Agents - Flow Fields
3// Particles follow a vector field defined by layered sine functions
4 
5state particles = []
6state time = 0.0
7state field_time = 0.0
8 
9let delta = dt()
10let sw = float(screen_width())
11let sh = float(screen_height())
12let PI = pi()
13time += delta
14field_time += delta * 0.3
15 
16let num_particles = 500
17let field_scale = 0.008
18 
19// Initialize particles
20if len(particles) == 0 then
21    for i in range(0, num_particles) do
22        push(particles, {
23            x: random(0.0, sw),
24            y: random(0.0, sh),
25            prev_x: random(0.0, sw),
26            prev_y: random(0.0, sh),
27            speed: random(40.0, 100.0),
28            hue: random(0.0, 1.0)
29        })
30    end
31end
32 
33// Flow field function - returns angle at position
34fn field_angle(px, py, t)
35    let n = sin(px * field_scale + t) * cos(py * field_scale * 0.7 + t * 0.5)
36    n += sin((px + py) * field_scale * 0.5 + t * 0.8) * 0.5
37    n += cos(px * field_scale * 1.5 - py * field_scale + t * 0.3) * 0.3
38    n * PI * 2.0
39end
40 
41// Update particles
42let new_particles = []
43for p in particles do
44    let angle = field_angle(p.x, p.y, field_time)
45 
46    let vx = cos(angle) * p.speed
47    let vy = sin(angle) * p.speed
48 
49    let nx = p.x + vx * delta
50    let ny = p.y + vy * delta
51 
52    let alive = true
53    // Respawn if off screen
54    if nx < 0.0 || nx > sw || ny < 0.0 || ny > sh then
55        nx = random(0.0, sw)
56        ny = random(0.0, sh)
57    end
58 
59    push(new_particles, {
60        x: nx, y: ny,
61        prev_x: p.x, prev_y: p.y,
62        speed: p.speed,
63        hue: p.hue
64    })
65end
66particles = new_particles
67 
68// === Drawing ===
69 
70// Semi-transparent background for trail effect (draw dark rect with low opacity)
71// Since we can't do alpha, we use a very dark rect each frame for fading trails
72draw_rect(0, 0, int(sw), int(sh), 5, 5, 12)
73 
74// Draw particles as short lines (from previous to current position)
75for p in particles do
76    // Color based on hue + position
77    let h = p.hue + field_angle(p.x, p.y, field_time) * 0.05
78    let hmod = ((h * 6.0) % 6.0 + 6.0) % 6.0
79    let cr = 0
80    let cg = 0
81    let cb = 0
82 
83    if hmod < 1.0 then cr = 255; cg = int(hmod * 255.0); cb = 50
84    elsif hmod < 2.0 then cr = int((2.0 - hmod) * 255.0); cg = 255; cb = 50
85    elsif hmod < 3.0 then cr = 50; cg = 255; cb = int((hmod - 2.0) * 255.0)
86    elsif hmod < 4.0 then cr = 50; cg = int((4.0 - hmod) * 255.0); cb = 255
87    elsif hmod < 5.0 then cr = int((hmod - 4.0) * 200.0) + 50; cg = 50; cb = 255
88    else cr = 255; cg = 50; cb = int((6.0 - hmod) * 200.0) + 50
89    end
90 
91    draw_line(int(p.prev_x), int(p.prev_y), int(p.x), int(p.y), cr, cg, cb)
92    draw_circle(int(p.x), int(p.y), 1, min(255, cr + 50), min(255, cg + 50), min(255, cb + 50))
93end
94 
95// Title (with background for readability)
96draw_rect(5, 5, 200, 50, 0, 0, 0)
97draw_text("Flow Field", 10, 10, 16, 100, 100, 120)
98draw_text("Particles follow a vector field", 10, 30, 12, 60, 60, 80)