code: private int getDoubleBondConfiguration2D(IBond bond, Point2d a, Point2d b, Point2d c, Point2d d)
              throws CDKException {
          if (bond.getOrder() != IBond.Order.DOUBLE) {
              return 0;
          }
          // no 2D coordinates or existing configuration
          if (a == null || b == null || c == null || d == null) return 0;
          Point2d cb = new Point2d(c.x - b.x, c.y - b.y);
          Point2d xT = new Point2d(cb.x - 1, cb.y);
          a.y = a.y - b.y - xT.y;
          d.y = d.y - b.y - xT.y;
          if ((a.y > 0 && d.y > 0) || (a.y < 0 && d.y < 0)) {
              return 5;
          } else {
              return 6;
          }
      }
docs: Gets the doubleBondConfiguration2D attribute of the AtomPlacer3D object
  using existing 2D coordinates.
  
  @param  bond           the double bond
  @param  a              coordinates (Point2d) of atom1 connected to bond
  @param  b              coordinates (Point2d) of atom2 connected to bond
  @param  c              coordinates (Point2d) of atom3 connected to bond
  @param  d              coordinates (Point2d) of atom4 connected to bond
  @return                The doubleBondConfiguration2D value