Procedural color palettes

float s = chf('scale');
float r = chf('rotate');
float a = chf('angle'); 
float b = chf('brightness');

function vector color(vector pos; float b){
    float hue = atan(pos.x, pos.y);
    hue = fit(hue, -PI, PI, 0.0, 1.0);
    float sat = length(pos);
    float val = b;
    vector clr = hsvtorgb(hue, sat, val);
    return clr;
}

function vector arc(float u, sc, rot, angle){
    vector2 dir = sample_circle_edge_uniform(rot);
    vector2 uv = set(u, sc);
    vector2 pos = sample_circle_slice(dir, angle*PI_2, uv);
    vector p = vector(pos);
    return p;
}

float u = i@primnum/(i@numprim - 1.0);
v@Cd = color(arc(u, s, r, a), b);

function vector recolor(vector clr){
    vector c = rgbtohsv(clr);
    c[1] = min(c[1], 0.75);
    c[2] = clamp(c[2], 0.05, 0.8);
    c = hsvtorgb(c);
    return c;
}

int seed = chi('seed');

vector a = rand(0, seed);
vector b = rand(1, seed);

a = recolor(a);
b = recolor(b);

int class = i@primnum % 3;
if(class == 1) b = a;
if(class == 2) a = b;

v@Cd = min(a, b) / pow(max(a, b), 0.25);

float fall = chf('falloff');

vector p = v@P * sqrt(1.5);

float wheel = atan2(p.y, p.x);
float hue = fit(wheel, -PI, PI, 0.0, 1.0);
float radius = length(p);

v@Cd = hsvtorgb(hue, 1.0 - pow(radius / 2.0, fall), radius);