Posted by gouessej on Feb 15, 2016; 4:23pm
URL: https://forum.jogamp.org/JOGL-2-support-for-Ardor3D-JMonkeyEngine-3-jzy3d-and-NiftyGUI-tp1706747p4036244.html

You have to use a blend state with a transparent render queue (RenderBucketType.Transparent), see Renanse's excellent answer:
http://stackoverflow.com/a/10264155