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What Is OpenGL

Thảo luận trong 'Ngoại ngữ' bắt đầu bởi bmnhy, 13 Tháng một 2007.

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    Tiêu đề" Chapter 1
    Nội dung:
    Introduction to OpenGL
    Chapter Objectives
    After reading this chapter, you'll be able to do the following:

    Appreciate in general terms what OpenGL offers

    Identify different levels of rendering complexity

    Understand the basic structure of an OpenGL program

    Recognize OpenGL command syntax

    Understand in general terms how to animate an OpenGL program

    This chapter introduces OpenGL. It has the following major sections:
    "What Is OpenGL?" explains what OpenGL is, what it does and doesn't do, and how it works.

    "A Very Simple OpenGL Program" presents a small OpenGL program and briefly discusses it. This section also defines a few basic computer-graphics terms.

    "OpenGL Command Syntax" explains some of the conventions and notations used by OpenGL commands.

    "OpenGL as a State Machine" describes the use of state variables in OpenGL and the commands for querying, enabling, and disabling states.

    "OpenGL-related Libraries" describes sets of OpenGL-related routines, including an auxiliary library specifically written for this book to simplify programming examples.

    "Animation" explains in general terms how to create pictures on the screen that move, or animate.


    What Is OpenGL?

    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. See "OpenGL-related Libraries" for more information about Open Inventor.)

    Now that you know what OpenGL doesn't do, here's what it does do. Take a look at the color plates - they illustrate typical uses of OpenGL. They show the scene on the cover of this book, drawn by a computer (which is to say, rendered) in successively more complicated ways. The following paragraphs describe in general terms how these pictures were made.

    Figure J-1 shows the entire scene displayed as a wireframe model - that is, as if all the objects in the scene were made of wire. Each line of wire corresponds to an edge of a primitive (typically a polygon). For example, the surface of the table is constructed from triangular polygons that are positioned like slices of pie.
    Note that you can see portions of objects that would be obscured if the objects were solid rather than wireframe. For example, you can see the entire model of the hills outside the window even though most of this model is normally hidden by the wall of the room. The globe appears to be nearly solid because it's composed of hundreds of colored blocks, and you see the wireframe lines for all the edges of all the blocks, even those forming the back side of the globe. The way the globe is constructed gives you an idea of how complex objects can be created by assembling lower-level objects.

    Figure J-2 shows a depth-cued version of the same wireframe scene. Note that the lines farther from the eye are dimmer, just as they would be in real life, thereby giving a visual cue of depth.

    Figure J-3 shows an antialiased version of the wireframe scene. Antialiasing is a technique for reducing the jagged effect created when only portions of neighboring pixels properly belong to the image being drawn. Such jaggies are usually the most visible with near-horizontal or near-vertical lines.

    Figure J-4 shows a flat-shaded version of the scene. The objects in the scene are now shown as solid objects of a single color. They appear "flat" in the sense that they don't seem to respond to the lighting conditions in the room, so they don't appear smoothly rounded.

    Figure J-5 shows a lit, smooth-shaded version of the scene. Note how the scene looks much more realistic and three-dimensional when the objects are shaded to respond to the light sources in the room; the surfaces of the objects now look smoothly rounded.

    Figure J-6 adds shadows and textures to the previous version of the scene. Shadows aren't an explicitly defined feature of OpenGL (there is no "shadow command"), but you can create them yourself using the techniques described in Chapter 13 . Texture mapping allows you to apply a two-dimensional texture to a three-dimensional object. In this scene, the top on the table surface is the most vibrant example of texture mapping. The walls, floor, table surface, and top (on top of the table) are all texture mapped.

    Figure J-7 shows a motion-blurred object in the scene. The sphinx (or dog, depending on your Rorschach tendencies) appears to be captured as it's moving forward, leaving a blurred trace of its path of motion.

    Figure J-8 shows the scene as it's drawn for the cover of the book from a different viewpoint. This plate illustrates that the image really is a snapshot of models of three-dimensional objects.

    The next two color images illustrate yet more complicated visual effects that can be achieved with OpenGL:
    Figure J-9 illustrates the use of atmospheric effects (collectively referred to as fog) to show the presence of particles in the air.

    Figure J-10 shows the depth-of-field effect, which simulates the inability of a camera lens to maintain all objects in a photographed scene in focus. The camera focuses on a particular spot in the scene, and objects that are significantly closer or farther than that spot are somewhat blurred.

    The color plates give you an idea of the kinds of things you can do with the OpenGL graphics system. The next several paragraphs briefly describe the order in which OpenGL performs the major graphics operations necessary to render an image on the screen. Appendix A, "Order of Operations" describes this order of operations in more detail.
    Construct shapes from geometric primitives, thereby creating mathematical descriptions of objects. (OpenGL considers points, lines, polygons, images, and bitmaps to be primitives.)

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

    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.

    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

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