GLSL (OpenGL Shading Language) is one of several commonly used shading languages for real-time rendering (other examples are Cg and HLSL). 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. GLSL is one of them. In fact, it is the shading language of several 3D graphics APIs (application programming interfaces), namely OpenGL, OpenGL ES 2.x, and WebGL. Therefore, GLSL is commonly used in applications for desktop computers, mobile devices, and the web.
About this Wikibook
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 GLSL within the game engine Unity.
Chapters 1 to 8 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 GLSL or Unity you should at least read through the tutorials in Chapter 1, “Basics”.
More details about the OpenGL pipeline and GLSL syntax in general are included in an “Appendix on the OpenGL Pipeline and GLSL Syntax”. Readers who are not familiar with OpenGL or GLSL might want to at least skim this part since a basic understanding of the OpenGL pipeline and GLSL syntax is very useful for understanding the tutorials.
About GLSL in Unity
GLSL programming in the game engine Unity is considerably easier than GLSL programming for an OpenGL, OpenGL ES, or WebGL 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 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, Windows users have to use the command-line argument
-force-opengl when starting Unity in order to be able to use GLSL shaders; for example, by changing the
Targetsetting in the properties of the desktop icon to:
"C:\Program Files\Unity\Editor\Unity.exe" -force-opengl. (On MacOS X, OpenGL and therefore GLSL is used by default.) Note that GLSL shaders cannot be used in Unity applications running in web browser on Windows.
- Secondly, this book assumes 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  (Unity Basics, Building Scenes, Asset Import and Creation).
- Furthermore, as of version 3.5, Unity supports a version of GLSL similar to version 1.0.x for OpenGL ES 2.0 (the specification is available at the “Khronos OpenGL ES API Registry”); however, Unity's shader documentation  focuses on shaders written in Unity's own “surface shader” format and Cg/HLSL . There are only very few details documented that are specific to GLSL shaders . Thus, this wikibook might also help to close some gaps in Unity's documentation. However, optimizations (see, for example, this blog) are usually not discussed.
Martin Kraus, August 2012Last modified on 17 August 2012, at 21:32