I don’t know what functionality niftyGUI has atm.
I can tell you that TonegodGUI accounts for this on the subscreen class by using an offscreen rendering surface to represent the GUI onto a quad. er… this conversation line would be applicable.
@pspeed’s Lemur is showcased, as well.
Nifty isn’t the only one, but it is the one that ships with Core SDK. I got my copy of Tonegod’s GUI from community modules in Tools/Plugins with the 3.0 downloadable stable.
So, until someone with Nifty experience shows up… I can only say how you would put a fullscreen quad on the screen to take advantage of linear interpolation of a 2D Texture for good ol’ unshaded.
“Common/MatDefs/Misc/Unshaded.j3md” in tonegod GUI