Cg教程：掌握实时图形编程

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"The Cg Tutorial - The Definitive Guide to Programmable Real-Time Graphics" 本书《The Cg Tutorial》由Randima Fernando和Mark J. Kilgard合著，是关于使用高级图形语言Cg进行可编程3D图形的绝佳教程。Cg语言在实质上与Direct3D的HLSL（高阶着色语言）非常相似。这本书提供了一种方法，使读者能够控制使用可编程图形硬件绘制的物体的形状、外观和运动，将这些属性的程序化控制与令人惊叹的实时图形效果相结合。 1.1 什么是Cg？ Cg是一种编程语言，它的设计目的是让开发者能够更深入地掌控3D图形的呈现。通过Cg，你可以编写程序来定义绘制的对象在图形硬件上的表现，包括它们的几何形状、颜色、纹理和动态效果。Cg使得艺术家和程序员可以创建出更为复杂和精细的视觉效果，尤其是在游戏、电影特效和虚拟现实应用中。 1.2 顶点、片段与图形管线现代图形硬件的数据流是通过一个称为图形管线的过程进行的，它包括多个阶段，如顶点处理、图元装配、光栅化和像素处理。Cg语言在这一过程中扮演了关键角色，特别是在顶点着色器和片段着色器阶段，允许开发者自定义顶点如何转换为屏幕空间，以及像素如何被着色。 1.3 Cg的历史发展 Cg的发展背景是图形技术不断演进的结果。随着GPU（图形处理器）的功能增强，对可编程性的需求也随之增加。Cg应运而生，旨在提供一个跨平台、易于使用的接口，使得开发者可以利用这些硬件的强大功能。 1.4 Cg环境使用Cg时，应用程序通过Cg运行时库与现有的3D应用程序编程接口（APIs）如OpenGL或Direct3D交互。这使得Cg程序能够在多种平台上工作，并能与各种图形库无缝集成，从而实现跨平台的可移植性。书中还详细介绍了如何设置和管理Cg程序，包括编译、链接和执行，以及如何处理错误和调试。此外，还涵盖了纹理映射、光照模型、动画和粒子系统等核心概念。读者将学习到如何使用Cg创建复杂的材质效果，模拟物理现象，以及实现高级视觉效果。《The Cg Tutorial》是学习和掌握3D图形编程的宝贵资源，无论是对于初学者还是有经验的开发者，都能从中受益匪浅，提升自己的图形编程技能。通过本书，读者不仅可以理解Cg语言，还能深入了解现代图形硬件的工作原理，从而更好地利用这些硬件实现令人震撼的实时图形效果。

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and Windows PCs, OpenGL is supported by Apple for its Macintosh personal computers. Linux users can use

either the Mesa open-source implementation of OpenGL or a hardware-accelerated implementation such as

NVIDIA's OpenGL driver for Linux. This flexibility makes OpenGL the industry's best cross-platform

programming interface for 3D graphics.

Over the last decade, OpenGL has evolved along with graphics hardware. OpenGL is extensible, meaning that

OpenGL implementers can add new functionality to OpenGL in an incremental way. Today, scores of OpenGL

extensions provide access to all the latest GPU features. This includes ARB-standardized extensions for vertex

and fragment programmability. As extensions are established, they are often rolled into the core OpenGL

standard so that the standard as a whole advances. At the time of this writing, the current version of OpenGL

is 1.4. Ongoing work to evolve OpenGL is underway in various OpenGL ARB working groups. This work

includes both assembly-level and high-level programmable interfaces. Because Cg operates as a layer above

such interfaces, it will continue to function with future revisions of OpenGL in a compatible manner.

Direct3D

Microsoft began developing the Direct3D programming interface about 1995 as part of its DirectX multimedia

initiative. Direct3D is one of the programming interfaces that make up DirectX. Microsoft introduced DirectX

and Direct3D to jump-start the consumer market for 3D graphics, particularly gaming, on Windows PCs.

Microsoft's Xbox game console also supports Direct3D. Direct3D is the most popular graphics API for games

on Windows, due to its history of closely matching the capabilities of available graphics hardware.

Every year or so, Microsoft has updated DirectX, including Direct3D, to keep up with the rapid pace of PC

hardware innovation. The current version of DirectX at the time of this writing is DirectX 9, which includes

HLSL, Microsoft's implementation of the same language syntax and constructs found in Cg.

3D Programming Interface D ente

A few years ago, OpenGL and Direct3D competed to see which programming interface would dominate,

particularly in the domain of Windows PCs. The competition continues to be good for both programming

interfaces, and each has improved in performance, quality, and functionality. In the area of GPU

programmability that Cg addresses, both programming interfaces have comparable capabilities. This is

because both OpenGL and Direct3D run on the same GPU hardware and the graphics hardware determines

the available functionality and performance. OpenGL has a slight advantage in functionality because hardware

vendors are better able to expose their entire feature set through OpenGL, though vendor-specific extensions

do add some complexity for developers.

Most software developers now choose a 3D programming interface based on programmer preference,

history, and their target market and hardware platform, rather than on technical grounds.

Cg supports either programming interface. You can write C g programs so that they work with either the

OpenGL or Direct3D programming interface. This is a huge boon for 3D content developers. They can pair

their 3D content with programs written in Cg and then render the content no matter what programming

interface the final application uses for 3D rendering.

1.4.2 The Cg Compiler and Runtime

No GPU can execute Cg programs directly from their textual form. A process known as compilation must

translate Cg programs into a form that the GPU can execute. The Cg compiler first translates your C g

program into a form accepted by the application's choice of 3D programming interface, either OpenGL or

Direct3D. Then your application transfers the OpenGL or Direct3D translation of your Cg program to the GPU

using the appropriate OpenGL or Direct3D commands. The OpenGL or Direct3D driver performs the final

translation into the hardware-executable form your GPU requires.

The details of this translation depend on the combined capabilities of the GPU and 3D programming interface.

How a Cg program compiles its intermediate OpenGL or Direct3D form depends on the type and generation of

GPU in your computer. It may be that your GPU is not capable of supporting a particular valid Cg program

because of limitations of the GPU itself. For example, your Cg fragment program will not compile if your

program accesses more texture units than your target GPU supports.

Support for Dynamic Compilation

When you compile a program with a conventional programming language such as C or C ++, compilation is an

offline process. Your compiler compiles the program into an executable that runs directly on the CPU. Once

compiled, your program does not need to be recompiled, unless you change the program code. We call this

static compilation.

C g is different because it encourages dynamic compilation, although static compilation is also supported. The

Cg compiler is not a separate program but part of a library known as the Cg runtime. 3D applications and

games using Cg programs must link with the C g runtime. Applications using C g then call C g runtime routines,

all prefixed with the letters

, to compile and manipulate Cg programs. Dynamic compilation allows C g

programs to be optimized for the particular model of GPU installed in the user's machine.

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CgGL and CgD3D, the 3D-API-Specific Cg Libraries

In addition to the core Cg runtime, Cg provides two closely related libraries. If your application uses OpenGL,

you will use the CgGL library to invoke the appropriate OpenGL routines to pass your translated Cg program

to the OpenGL driver. Likewise, if your application uses Direct3D, you will use the CgD3D library to invoke

the appropriate Direct3D routines to pass your translated Cg program to the Direct3D driver. Normally, you

would use either the CgGL or the C gD3D library, but not both, because most applications use either OpenGL

or Direct3D, not both.

Compared with the core C g runtime library that contains the C g compiler, the C gGL and C gD3D libraries are

relatively small. Their job is to make the appropriate OpenGL or Direct3D calls for you to configure Cg

programs for execution. These calls transfer a translated Cg program to the appropriate driver that will

further translate the program into a form your GPU can execute. For the most part, the CgGL and CgD3D

libraries have similar routines. The routines in the CgGL library begin with

cgGL

; the routines in the C gD3D

library begin with

cgD3D

How the Cg Runtime Fits into Your Application

Figure 1-13 shows how a typical 3D application uses the Cg libraries. If you are a programmer, you will want

to learn more about the Cg runtime and the specific library for the 3D API your application uses to render 3D

graphics. Most of this book focuses on the Cg language itself and on how to write C g programs, but Appendix

B has more information about the Cg runtime library.

Figure 1-13 How C g Fits into a Standard C g Application

1.4.3 The CgFX Toolkit and File Format

C g programs need 3D models, textures, and other data to operate on. A Cg program without any associated

data is useless. Cg programs and data also require the correct 3D programming interface configuration and

state. It is often helpful to have a way to bundle all the information required to render a 3D model, including

its associated Cg program.

What CgFX Provides

C gFX is a standardized file format for representing complete effects and appearances. As they did with Cg,

Microsoft and NVIDIA collaborated to develop the CgFX format. C gFX files are text-based, with a syntax that

is a superset of Cg's, and may contain any number of C g programs. The

.fx

suffix identifies CgFX files. A

CgFX file describes the complete render state for a particular effect: multiple passes, texture states, and any

number of individual vertex and fragment programs may be defined to create a complete appearance or

effect. An accompanying development toolkit is provided for using and parsing CgFX files. The toolkit exposes

user-interface hooks to host applications, so that CgFX-aware applications can automatically supply

meaningful controls and semantics to users and developers alike.

Cg programs describe the vertex or fragment processing that takes place in a single rendering pass, but

some complex shading algorithms require multiple rendering passes. CgFX offers a format to encode complex

multipass effects, including designating which C g program is used for each rendering pass.

More specifically, CgFX supports three additional capabilities beyond what the core C g language supports:

CgFX provides a mechanism for specifying multiple rendering passes and optional multiple

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implementations for a single effect.

CgFX allows you to specify nonprogrammable rendering states, such as alpha-test modes and

texture-filtering. The settings for these render states may take the form of simple expressions,

which are evaluated on the CPU when the effect is initialized.

CgFX allows annotations to be added to shaders and shader parameters. These annotations provide

additional information to applications, including content creation applications. For example, an

annotation can specify the allowed range of values for a shader parameter.

Multiple Shader Instancing

The CgFX file format encapsulates multiple implementations of C g programs for a given shader. This means

you can have one Cg shader program written for a third-generation or fourth-generation GPU, while also

including a simpler program that supports a less capable, second-generation GPU. An application loading the

CgFX file can determine at runtime the most appropriate shader implementation to use based on the

computer's available GPU.

Multiple instancing of C g programs with CgFX is one way to address the functional variations in GPUs of

different generations or different hardware vendors. Multiple instancing also lets you develop a Cg program

specialized for a particular 3D API—for example, if OpenGL exposes extra functionality through an extension.

Cg programs specialized for Direct3D, standard OpenGL, or OpenGL with extensions can all be contained in a

single CgFX file.

CgFX and Digital Content Creation

The CgFX Toolkit consists of the C gFX compiler, which supports the full C gFX syntax; the C gFX runtime API,

for loading and manipulating CgFX files; and plug-in modules for major digital content creation (DC C )

applications such as Alias|Wavefront's Maya and Discreet's 3ds max. Figure 1-14 shows these applications

making use of CgFX. Softimage|XSI 3.0 provides direct support for C g compilation in its Render Tree.

Figure 1-14 Digital C ontent C reation Applications That Use C g and C gFX

Prior to CgFX, there was no standard way for a DC C application to export 3D content with all the associated

shading knowledge necessary to render the content in real time. Now the major DCC applications use C gFX in

their content creation process and support the CgFX file format. This means that C gFX can significantly

improve the artistic workflow from DCC applications to real-time games and other 3D applications. Using

CgFX, artists can view and tweak C g shaders and associated 3D content to see, from within the DC C tool of

their choice, how their work will appear in a 3D game or application.

How CgFX Fits into Your Application

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Figure 1-15 shows how a CgFX file containing multiple instanced shaders is used by an application in

conjunction with the C g runtime and your choice of rendering API. Most of this book focuses on the Cg

language itself and on how to write Cg programs, rather than CgFX, but see Appendix C for more information

about the CgFX file format and its associated runtime API.

Figure 1-15 How C gFX Fits into a Standard Application

Onward to the Tutorial

Having finished this introduction to the C g programming language, you are now ready to take on the tutorial

chapters, which will teach you how to write Cg programs.

1.5 Exercises

The exercises at the end of each chapter help you review your knowledge and develop practical programming

skills.

Answer this: Name two standard 3D programming interfaces for which you can compile Cg

programs. What operating systems does each programming interface support?

Answer this: What are the major stages of the graphics pipeline? In what order are the stages

arranged?

Answer this: Where do vertex and fragment programs fit into the pipeline?

Answer this: What is a vertex? What is a fragment? Distinguish a fragment from a pixel.

Try this yourself: We haven't begun writing Cg programs yet (we'll get there soon enough in the

next chapter), so take a break and watch a good feature-length computer graphics animation such

as Monsters, Inc.

1.6 Further Reading

C g builds on a host of concepts in computer language design, computer hardware design, and computer

graphics. Doing justice to all these contributions in the context of this tutorial is not always practical. What we

attempt in the "Further Reading" section at the end of each chapter is to offer you pointers to learn more

about the contributions that underlie the topics in each chapter.

There are plenty of books on C. The C Programming Language, Third Edition (Prentice Hall, 2000), by Brian

Kernighan and Dennis Ritchie, is a classic; the authors invented the C language. C g includes concepts from

both C and C++. There now may actually be more books about C++ than about C . The classic C ++ book is

The C++ Programming Language, Third Edition (Addison-Wesley, 2000), by Bjarne Stroustrup, who invented

the language.

To learn more about the RenderMan Shading Language, read The RenderMan Companion: A Programmer's

Guide to Realistic Computer Graphics (Addison-Wesley, 1989), by Steve Upstill. Pat Hanrahan and Jim Lawson

published a SIGGRAPH paper about RenderMan called "A Language for Shading and Lighting Calculations"

(ACM Press) in 1990.

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Robert Cook's 1984 SIGGRAPH paper titled "Shade Trees" (AC M Press) motivated the development of

RenderMan.

The development of programmable graphics hardware and its associated languages has been an active and

fruitful research area for almost a decade. Anselmo Lastra, Steven Molnar, Marc Olano, and Yulan Wang at

UNC published an early research paper in 1995 titled "Real-Time Programmable Shading" (AC M Press).

Researchers at UNC also published several papers about their programmable PixelFlow graphics architecture.

Marc Olano and Anselmo Lastra published a SIGGRAPH paper titled "A Shading Language on Graphics

Hardware: The PixelFlow Shading System" (ACM Press) in 1998.

Kekoa Proudfoot, Bill Mark, Svetoslav Tzvetkov, and Pat Hanrahan published a SIGGRAPH paper in 2001

titled "A Real-Time Procedural Shading System for Programmable Graphics Hardware" (ACM Press) that

describes a GPU-oriented shading language developed at Stanford.

Real-Time Rendering, Second Edition (A. K. Peters, 2002), written by Eric Haines and Tomas Akenine-M?ller,

is an excellent resource for further information about graphics hardware and interactive techniques.

The OpenGL Graphics System: A Specification documents the OpenGL 3D programming interface. The best

tutorial for learning OpenGL programming is the OpenGL Programming Guide: The Official Guide to Learning

OpenGL, Third Edition (Addison-Wesley, 1999), by Mason Woo, Jackie Neider, Tom Davis, and Dave Shreiner.

The www.opengl.org Web site serves up much more information about OpenGL.

Documentation for the Direct3D programming interface is available from Microsoft's msdn.microsoft.com

Web site.

NVIDIA provides further information about the Cg runtime, C gFX, and C g itself on its Developer Web site at

developer.nvidia.com/Cg .

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