1.3. Execution Model

The OpenGL API is focused on drawing graphics into frame buffer memory and, to a lesser extent, in reading back values stored in that frame buffer. It is somewhat unique in that its design includes support for drawing threedimensional geometry (such as points, lines, and polygons, collectively referred to as PRIMITIVES) as well as for drawing images and bitmaps.

The execution model for OpenGL can be described as client-server. An application program (the client) issues OpenGL commands that are interpreted and processed by an OpenGL implementation (the server). The application program and the OpenGL implementation can execute on a single computer or on two different computers. Some OpenGL state is stored in the address space of the application (client state), but the majority of it is stored in the address space of the OpenGL implementation (server state).

OpenGL commands are always processed in the order in which they are received by the server, although command completion may be delayed due to intermediate operations that cause OpenGL commands to be buffered. Out-of-order execution of OpenGL commands is not permitted. This means, for example, that a primitive will not be drawn until the previous primitive has been completely drawn. This in-order execution also applies to queries of state and frame buffer read operations. These commands return results that are consistent with complete execution of all previous commands.

Data binding for OpenGL occurs when commands are issued, not when they are executed. Data passed to an OpenGL command is interpreted when the command is issued and copied into OpenGL memory if needed. Subsequent changes to this data by the application have no effect on the data that is now stored within OpenGL.