BSP is one of the best node builders for Doom. What are Nodes? Doom uses a rendering algorithm based on a binary space partition, otherwise known as a BSP tree. This is stored in a data lump called NODES in the WAD file. So tools to build the BSP tree are known as node builders. Before you can play a level that you have created, you must use a node builder to create the data that Doom will use to render the level. BSP is one of several node builders that can do this.