GEM version has now the background window output too. For this to work, you'll need NVDI 2.51 (bindings programmed only for SozobonX C xgemfast library) or later for off-screen bitmaps (or alternatively you can try the freeware Enhancer TSR). For simple image output this isn't necessary though.
Note: W library is almost as easy to program as Grafix and more complete / better designed. So now I'm joining grafix lib X version with Wlib to get Wlib emulation over Xlib. After project completes, one can use Kay Römer's W toolkit when programming anything major. (1st October 1996)