code: public void unproject(double x, double y, double z, double[] out) {
      glu.gluUnProject(x, y, z, modelview, 0, projection, 0, viewp, 0, out, 0);
    }
docs: Unproject a screen coordinate (at depth 0) to 3D model coordinates.
  
  @param x X
  @param y Y
  @param z Z
  @param out output buffer