The only shading methods provided by OpenGL are local. That is, methods for determining surface color such as ray-tracing or radiosity that require obtaining information from other parts of the scene are not directly supported. The reason is that such methods require knowledge of the global scene database, but so far specialized graphics hardware is structured as a pipeline of localized operations and does not provide facilities to store and traverse the large amount of data necessary to represent a complex scene. Global shading methods may be used with OpenGL only if the shading can be precomputed and the results associated with graphical objects before they are transmitted to OpenGL.