Selection

　　Selection is a powerful feature of OpenGL that allows you click at some position of the OpenGL window using the mouse and determine which of your objects lie beneath it. The act of selecting a specific object is called Picking. With OpenGL's selection feature, we can specify a viewing volume and determine which objects fall within that viewing volume. A powerful utility function, gluPickMatrix, produces a matrix which can be used to create a smaller viewing volume placed beneath the mouse cursor. Then we use selection to test this viewing volume to see which objects are contained by it.

　　Selection is actually a rendering mode, but in this mode no pisels are actually copied onto the frame buffer. Instead, primitives drawn within the viewing volume produce hit records in a selection buffer. We must set up this selection buffer in advance and name the primitives or groups of primitives so that they can be identified in the selection buffer.We can then parse the buffer to determine which objects intersected the viewing volume.

　　Naming Primitives

　　We have to name a group of primitives such as one describing a cube or a cylinder etc in order to identify them. These names are nothing but integers such as for display list names. The names list is maintained on the named stack. After we initialize the name stack we can push names on the stack or simply replace the name currently on the top of the stack. When a hit occurs during selection, all the names on the stack are copied onto the selection buffer.