code: public Frustum setToPerspective (double fovy, double aspect, double znear, double zfar) {
          double top = znear * Math.tan(fovy / 2f), bottom = -top;
          double right = top * aspect, left = -right;
          return setToFrustum(left, right, bottom, top, znear, zfar);
      }
docs: Sets this frustum to one pointing in the Z- direction with the specified parameters
  determining its size and shape (see the OpenGL documentation for
  <code>gluPerspective</code>).
  
  @param fovy the vertical field of view, in radians.
  @param aspect the aspect ratio (width over height).
  @param znear the distance to the near clip plane.
  @param zfar the distance to the far clip plane.
  @return a reference to this frustum, for chaining.