What Is OpenGL?

OpenGL is a software interface to graphics hardware. This interface consists of about 120 distinct commands, which you use to specify the objects and operations needed to produce interactive three-dimensional applications.

OpenGL is designed to work efficiently even if the computer that displays the graphics you create isn't the computer that runs your graphics program. This might be the case if you work in a networked computer environment where many computers are connected to one another by wires capable of carrying digital data. In this situation, the computer on which your program runs and issues OpenGL drawing commands is called the client, and the computer that receives those commands and performs the drawing is called the server. The format for transmitting OpenGL commands (called the protocol) from the client to the server is always the same, so OpenGL programs can work across a network even if the client and server are different kinds of computers. If an OpenGL program isn't running across a network, then there's only one computer, and it is both the client and the server.

OpenGL is designed as a streamlined, hardware-independent interface to be implemented on many different hardware platforms. To achieve these qualities, no commands for performing windowing tasks or obtaining user input are included in OpenGL; instead, you must work through whatever windowing system controls the particular hardware you're using. Similarly, OpenGL doesn't provide high-level commands for describing models of three-dimensional objects. Such commands might allow you to specify relatively complicated shapes such as automobiles, parts of the body, airplanes, or molecules. With OpenGL, you must build up your desired model from a small set of geometric primitive - points, lines, and polygons. (A sophisticated library that provides these features could certainly be built on top of OpenGL - in fact, that's what Open Inventor is)

The next several paragraphs briefly describe the order in which OpenGL performs the major graphics operations necessary to render an image on the screen.

1. Construct shapes from geometric primitives, thereby creating mathematical descriptions of objects. (OpenGL considers points, lines, polygons, images, and bitmaps to be primitives.)

2. Arrange the objects in three-dimensional space and select the desired vantage point for viewing the composed scene.

3. Calculate the color of all the objects. The color might be explicitly assigned by the application, determined from specified lighting conditions, or obtained by pasting a texture onto the objects.

4. Convert the mathematical description of objects and their associated color information to pixels on the screen. This process is called rasterization.

During these stages, OpenGL might perform other operations, such as eliminating parts of objects that are hidden by other objects (the hidden parts won't be drawn, which might increase performance). In addition, after the scene is rasterized but just before it's drawn on the screen, you can manipulate the pixel data if you want.

From : "OpenGL Programming Guide"