code: boolean left(float[] a, float[] b, float[] c) {
          float u1 = b[0] - a[0];
          float v1 = b[2] - a[2];
          float u2 = c[0] - a[0];
          float v2 = c[2] - a[2];
          return u1 * v2 - v1 * u2 < 0;
      }
docs: Returns true if 'c' is left of line 'a'-'b'.