# Cg Programming/Unity/Introduction

Nvidia's programming language Cg (C for graphics) is one of several commonly used shading languages for real-time rendering (other examples are Direct3D's HLSL and OpenGL's GLSL). These shading languages are used to program shaders (i.e. more or less small programs) that are executed on a GPU (graphics processing unit), i.e. the processor of the graphics system of a computer – as opposed to the CPU (central processing unit) of a computer.

GPUs are massively parallel processors, which are extremely powerful. Most of today's real-time graphics in games and other interactive graphical applications would not be possible without GPUs. However, to take full advantage of the performance of GPUs, it is necessary to program them directly. This means that small programs (i.e. shaders) have to be written that can be executed by GPUs. The programming languages to write these shaders are shading languages. Cg is one of them. In fact, it was one of the first high-level shading languages for GPUs and is implemented for several 3D graphics APIs (application programming interfaces), most importantly OpenGL and Direct3D. Today, the main reason for its popularity is its similarity to HLSL, which is the shading language of Microsoft's Direct3D. In practice, there is usually no difference between Cg and HLSL shaders.

This wikibook was written with students in mind, who like neither programming nor mathematics. The basic motivation for this book is the observation that students are much more motivated to learn programming environments, programming languages and APIs if they are working on specific projects. Such projects are usually developed on specific platforms and therefore the approach of this book is to present Cg within the game engine Unity.

Chapters 1 to 9 of the book consist of tutorials with working examples that produce certain effects. Note that these tutorials assume that you read them in the order in which they are presented, i.e. each tutorial will assume that you are familiar with the concepts and techniques introduced by previous tutorials. If you are new to Cg or Unity you should at least read through the tutorials in Chapter 1, “Basics”.

More details about the programmable graphics pipeline and Cg syntax in general are included in an “Appendix on the Programmable Graphics Pipeline and Cg Syntax”. Readers who are not familiar with GPUs or Cg might want to at least skim this part since a basic understanding of this pipeline and Cg syntax is very useful for understanding the tutorials.

Cg programming in the game engine Unity is considerably easier than Cg programming for an OpenGL or Direct3D application. Import of meshes and images (i.e. textures) is supported by a graphical user interface; mipmaps and normal maps can be computed automatically; the most common vertex attributes and uniforms are predefined; OpenGL and Direct3D states can be set by very simple commands; etc.

A free version of Unity can be downloaded for Windows and MacOS at Unity's download page. All of the included tutorials work with the free version. Three points should be noted:

• First, the tutorials assume that readers are somewhat familiar with Unity. If this is not the case, readers should consult the first three sections of Unity's User Guide [1] (Unity Basics, Building Scenes, Asset Import and Creation).
• Second, Unity doesn't distinguish between Cg (the shading language by Nvidia) and HLSL (the shading language in Direct3D) since the two languages are very similar; thus, most of these tutorials also apply to HLSL.
• Furthermore, Cg is documented by Nvidia's Cg Tutorial and Nvidia's Cg Language Specification. However, these descriptions are missing the details specific to Unity. On the other hand, Unity's shader documentation [2] focuses on shaders written in Unity's own “surface shader” format, while the documentation of shaders in Cg/HLSL is very limited [3]. Thus, learning Cg programming in Unity without prior knowledge of Cg can be rather difficult. This wikibook tries to close this gap by providing an introduction to Cg programming in Unity without requiring prior knowledge of Cg.

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Martin Kraus, March 2013