Reduced Z-buffer generating method, hidden surface removal method and occlusion culling method

A rendering processing apparatus is provided which performs occlusion culling for excluding from rendering targets a hidden object behind another object as seen from a point of view, when given a plurality of objects. An object input unit stores a plurality of objects in an object storing unit. An internal volume generating unit generates an internal volume which is included in a target object. A reduced Z-buffer updating unit updates a reduced Z-buffer based on the internal volume. An external volume generating unit generates an external volume which includes the target object subject to culling test. A culling determination unit consults the reduced Z-buffer and performs a Z culling test on the target object based on the external volume.