x els ). ▁ ▁For ▁several ▁years , ▁Ken ▁worked ▁on ▁a ▁modern ▁engine ▁based ▁entirely ▁on ▁vo x els , ▁known ▁as ▁V ox lap . ▁ ▁Room ▁over ▁room ▁ ▁One ▁limitation ▁of ▁the ▁Build ▁engine ▁is ▁that ▁its ▁level ▁geometry ▁is ▁only ▁capable ▁of ▁representing ▁one ▁connection ▁between ▁se ctors ▁for ▁any ▁given ▁wall . ▁Due ▁to ▁this , ▁a ▁structure ▁as ▁simple ▁as ▁a ▁sh elf ▁with ▁space ▁both ▁above ▁and ▁below ▁it ▁is ▁impossible , ▁though ▁sometimes ▁spr ites ▁or ▁vo x els ▁can ▁be ▁subst it uted . ▁Build ings ▁with ▁several ▁flo ors ▁are ▁techn ically ▁possible , ▁but ▁it ▁is ▁not ▁possible ▁for ▁such ▁a ▁building ▁to ▁contain ▁an ▁external ▁window ▁directly ▁above ▁or ▁below ▁another ▁window . ▁In ▁addition , ▁some ▁libert ies ▁will ▁need ▁to ▁be ▁taken ▁with ▁the ▁st air cases , ▁elev ators , ▁and ▁other ▁methods ▁of ▁access ▁for ▁each ▁floor . ▁ ▁Several ▁Build ▁engine ▁games ▁( nam ely ▁Sh adow ▁War rior , ▁Blood , ▁and ▁Red ne ck ▁R amp age ) ▁worked ▁around ▁this ▁by ▁displaying ▁a ▁" view port " ▁to ▁another ▁sector ▁through ▁an ▁additional ▁rendering ▁pass . ▁This ▁technique , ▁called ▁room - over - room ▁( ROR ), ▁appears ▁se am less ▁to ▁the ▁player . ▁In ▁addition ▁to ▁an ▁expanded ▁range ▁of ▁vertical ▁construction , ▁R OR ▁was ▁often ▁used ▁to ▁give ▁bodies ▁of ▁water ▁transl uc ent ▁surfaces . ▁R OR ▁was ▁never ▁a ▁feature ▁of ▁the ▁Build ▁engine ▁itself