OpenGL provides access to rendering operations only. There are no facilities for obtaining user input from such devices as keyboards and mice, since it is expected that any system (in particular, a window system) under which OpenGL runs must already provide such facilities. Further, the effects of OpenGL commands on the framebuffer are ultimately controlled by the window system (if there is one) that allocates framebuffer resources. The window system determines which portions of the framebuffer OpenGL may access and communicates to OpenGL how those portions are structured. These considerations make OpenGL window system independent.