▁( also ▁called ▁off line ▁rendering ). ▁Real - time ▁rendering ▁is ▁used ▁to ▁interact ively ▁render ▁a ▁scene , ▁like ▁in ▁ 3 D ▁computer ▁games , ▁and ▁generally ▁each ▁frame ▁must ▁be ▁rendered ▁in ▁a ▁few ▁mill iseconds . ▁Off line ▁rendering ▁is ▁used ▁to ▁create ▁real istic ▁images ▁and ▁mov ies , ▁where ▁each ▁frame ▁can ▁take ▁hours ▁or ▁days ▁to ▁complete , ▁or ▁for ▁debugging ▁of ▁complex ▁graphics ▁code ▁by ▁program mers . ▁ ▁Real - time ▁software ▁rendering ▁For ▁real - time ▁rendering ▁the ▁focus ▁is ▁on ▁performance . ▁The ▁earliest ▁texture ▁mapped ▁real - time ▁software ▁render ers ▁for ▁PC s ▁used ▁many ▁tr icks ▁to ▁create ▁the ▁ill usion ▁of ▁ 3 D ▁geometry ▁( true ▁ 3 D ▁was ▁limited ▁to ▁flat ▁or ▁G ou ra ud - sh aded ▁pol yg ons ▁employed ▁mainly ▁in ▁flight ▁sim ul ators .) ▁Ult ima ▁Under world , ▁for ▁example , ▁allowed ▁a ▁limited ▁form ▁of ▁looking ▁up ▁and ▁down , ▁sl anted ▁flo ors , ▁and ▁rooms ▁over ▁rooms , ▁but ▁res orted ▁to ▁spr ites ▁for ▁all ▁detailed ▁objects . ▁The ▁technology ▁used ▁in ▁these ▁games ▁is ▁currently ▁categor ized ▁as ▁ 2 . 5 D . ▁ ▁One ▁of ▁the ▁first ▁games ▁architect ur ally ▁similar ▁to ▁modern ▁ 3 D ▁titles , ▁allowing ▁full ▁ 6 Do F , ▁was ▁Des cent , ▁which ▁featured ▁ 3 D ▁models ▁entirely ▁made ▁from ▁bitmap ▁text ured ▁tri angular ▁pol yg ons . ▁V ox