Thanks for your encouraging words Andreas, but this image was just made for fun for the 2D theme, I had never a thought to post it to my gallery (we have a lot of fun with our theme images this month in 2D) The reason I posted it here was theoretical, and I have got answers, thanks to you, Seadreamer and tresamie. Maybe I have to give UF a fair chance, but I hate reading manuals :-)