.BSP is a map file extension used in games such as the Quake series, and games that use the Source engine, such as the Half-Life series and Portal series. .BSP files use binary space partitioning to generate levels that can be rendered quickly while minimizing the number of polygons that need to be redrawn every time the screen refreshes. One of the biggest problems with the .BSP file format is that large open areas do not work well due to the nature of the partitioning algorithm used.