GlueDrinker has asked me if the idtech3 engine supports different ground planes and if it is possible to set them in GtkRadiant to obtain an effect like this :
For me the answer is no also because he makes me look at a real example for the idtech3 engine (
http://www.moddb.com/games/quake-iii-arena/addons/relativitypk3 ) that was made by "cheating" as admitted by its author, but it would be appreciated to have a more detailed explanation or eventually to deny our conclusion.
PS: feel free to remove the image and the link if not allowed