docs: public static double[] line_of_twoP(double[] p1, double[] p2) {
      // kb[0]:if the line is vertical;kb[1]:a value if kb[0]=1,k value or else;kb[2]:insignificance if kb[0]=1,b value or else;
      double[] kb = new double[3];
      if (p1[0] == p2[0]) {
        kb[0] = 1;
        kb[1] = p1[0];
        kb[2] = 0;
      } else {
        kb[0] = 0;
        kb[1] = (p1[1] - p2[1]) / (p1[0] - p2[0]);
        kb[2] = p1[1] - kb[1] * p1[0];
      }
      return kb;
    }