float shape(vec2 p, int N, float size){
    p = p * 2. - 1.;

    float a = atan(p.x, p.y) + PI;
    float r = TWO_PI / float(N);
    float l = length(p);

    float d = cos(floor(.5 + a / r) * r - a) * l;

    return step(size, d);
  }