code: function (r, g, b) {
      var k = Math.min(1 - r, 1 - g, 1 - b),
        l = 1 - k;
      if (k == 1) return [0, 0, 0, -k / 255];
      else return [(1 - r - k) / l, (1 - g - k) / l, (1 - b - k) / l, -k / 255];
    }
docs: attribute definitions