anti-aliased checker-sphere procedural glsl pattern

checker-sphere.glsl

float checkerSphere(){
    vec3 normal = normalize(vNormal);
    float c = (acos(normal.y)/PI)*20.0;
    float dc = fwidth(c);
    c = mod(c, 2.0);
    c = smoothstep(0.5-dc, 0.5+dc, c) - smoothstep(1.5-dc, 1.5+dc, c);

    vec2 dir = normalize(normal.xz);
    float t = (atan(dir.x, dir.y)/TAU)*40.0;
    float dt = fwidth(t);
    t = mod(t, 2.0);
    t = smoothstep(0.5-dt, 0.5+dt, t) - smoothstep(1.5-dt, 1.5+dt, t);
    t = mix(t, 1.0-t, c);
    t = t*0.1+0.5;

    return t;
}