Walkthrough applications based on portal

Portal as one of the visibility determination methods is widely used for large scale indoor scene. With the deepening research, we find that portal can do more works than we thought, including simulating mirror, connecting cells and dividing cells more simply. In this paper, first, we give the basic conception of portal, analyze the status and development trend of it, and divide portal into two categories: conventional portal and arbitrary portal. Conventional portal is a planar portal, defined on cell boundaries and used to form a CPG graph connecting the cells of the subdivision. Arbitrary portal is non-planar portal, can be any arbitrary surface, so it may exist anywhere within a cell or outside the indoor scene and connect to any space with an adjacent cell. Then, we describe the detail development of each type, comparing their difference and the different methods about portal. Finally, we point out some key research issues that need to be studied in the future.