let mut glfw_ctx = GlfwContext::new(900, 700)?;

init(Config::default())?;

let surface = glfw_ctx.create_surface(900, 700)?;

let graphics = graphics_create(surface, 900, 700, TextureFormat::Rgba16Float)?;

graphics_mode_3d(graphics)?;

transform_set_position(graphics, Vec3::new(0.0, 2.0, 14.0))?;

transform_look_at(graphics, Vec3::new(0.0, 0.0, 0.0))?;

let sphere = geometry_sphere(0.1, 8, 6)?;

let capacity: u32 = 800;

let position_attr = geometry_attribute_position();

let color_attr = geometry_attribute_color();

let scale_attr = geometry_attribute_scale();

let dead_attr = geometry_attribute_dead();

let age_attr = geometry_attribute_create("age", AttributeFormat::Float)?;

let p = particles_create(

capacity,

vec![

position_attr,

color_attr,

scale_attr,

dead_attr,

age_attr,

],

)?;

let dead_buf = particles_buffer(p, dead_attr)?

.ok_or(error::ProcessingError::ParticlesNotFound)?;

let color_buf = particles_buffer(p, color_attr)?

.ok_or(error::ProcessingError::ParticlesNotFound)?;

// Mark all slots dead initially so the unemitted ring slots don't render.

let init_dead: Vec<u8> = (0..capacity)

.flat_map(|_| 1.0_f32.to_le_bytes())

.collect();

buffer_write(dead_buf, init_dead)?;

let mat = { let m = material_create_unlit()?; material_set_albedo_buffer(m, color_buf)?; m };

let aging_shader = shader_create(AGING_SHADER)?;

let aging = compute_create(aging_shader)?;

// burst × (ttl × 60) ≈ steady-state alive count (~360 here, well under capacity 800).

let burst: u32 = 6;

let dt: f32 = 1.0 / 60.0;

let ttl: f32 = 1.0;

let mut frame: u32 = 0;

while glfw_ctx.poll_events() {

graphics_begin_draw(graphics)?;

graphics_record_command(

graphics,

DrawCommand::BackgroundColor(bevy::color::Color::srgb(0.04, 0.04, 0.07)),

)?;

graphics_record_command(graphics, DrawCommand::Material(mat))?;

graphics_record_command(

graphics,

DrawCommand::Particles { particles: p, geometry: sphere },

)?;

graphics_end_draw(graphics)?;

// Spawn `burst` new particles per frame in a small fountain.

let mut positions: Vec<f32> = Vec::with_capacity(burst as usize * 3);

let mut colors: Vec<f32> = Vec::with_capacity(burst as usize * 4);

for k in 0..burst {

let i = frame * burst + k;

// Cheap pseudo-random offset.

let u = ((i.wrapping_mul(2654435761) >> 8) & 0xFFFF) as f32 / 65535.0;

let v = ((i.wrapping_mul(40503) >> 8) & 0xFFFF) as f32 / 65535.0;

let theta = u * std::f32::consts::TAU;

let r = v * 0.6;

positions.push(theta.cos() * r);

positions.push(2.5);

positions.push(theta.sin() * r);

let h = (i as f32 * 0.013) % 1.0;

let (cr, cg, cb) = hsv_to_rgb(h, 0.85, 1.0);

colors.push(cr);

colors.push(cg);

colors.push(cb);

colors.push(1.0);

}

let position_bytes: Vec<u8> = positions.iter().flat_map(|f| f.to_le_bytes()).collect();

let color_bytes: Vec<u8> = colors.iter().flat_map(|f| f.to_le_bytes()).collect();

let zero_floats: Vec<u8> = (0..burst).flat_map(|_| 0.0_f32.to_le_bytes()).collect();

// Reset scale to 1 for newly emitted particles (the aging shader will

// shrink them as age progresses).

let one_scale: Vec<u8> = (0..burst)

.flat_map(|_| {

[1.0_f32, 1.0, 1.0]

.iter()

.flat_map(|f| f.to_le_bytes())

.collect::<Vec<u8>>()

})

.collect();

particles_emit(

p,

burst,

vec![

(position_attr, position_bytes),

(color_attr, color_bytes),

(scale_attr, one_scale),

(age_attr, zero_floats.clone()),

(dead_attr, zero_floats),

],

)?;

// Age + drop + kill.

compute_set(

aging,

"params",

shader_value::ShaderValue::Float4([dt, ttl, 0.0, 0.0]),

)?;

particles_apply(p, aging)?;

frame += 1;

}

Ok(())

let i = (h * 6.0).floor();

let f = h * 6.0 - i;

let p = v * (1.0 - s);

let q = v * (1.0 - f * s);

let t = v * (1.0 - (1.0 - f) * s);

match (i as i32) % 6 {

0 => (v, t, p),

1 => (q, v, p),

2 => (p, v, t),

3 => (p, q, v),

4 => (t, p, v),

_ => (v, p, q),

}