Graphics Shaders, 2nd Edition

91

The GLSL Shader

Language

5

As shader capabilities in graphics hardware have become more exible, shader

languages have been developed to give the graphics programmer access to

these capabilities. The GLSL shading language was designed to be device inde-

pendent and has been part of the OpenGL standardfrom OpenGL 2.0 forward.

It accomplishes its device independence by having compilers built into the

graphics card driver translate the GLSL code into the specic device instruc-

tions for that card. The actual process of aaching shaders to shader programs,

compiling them, and linking them to be downloaded into the graphics card is

part of the GLSL API, covered in Chapter 14.

GLSL is a very C-like language, with most of the same fundamental

code structure and operators that are found in that language. Thus, there

are no challenges to the graphics programmer in understanding the control

ow, basic operations, or basic data types in the language. However, there

are some areas where GLSL extends the capabilities of C, some areas where

92

5. The GLSL Shader Language

GLSL omits some of the capabilities

of C, and some areas where GLSL

has language features that remind

us of the best of earlier generations

of computer languages. This chap-

ter focuses on these dierences and

discusses why they are needed for

the shader environment. There is a

tendency for any discussion like this

to have a strong avor of a language

manual, and you might nd that

you use this chapter more as a refer-

ence than as general reading.

We introduced a number

of GLSL language features in

Chapter 3, but here we take a more thorough approach to the language and

describe it more formally. We are working from the GLSL language specica-

tion [23] and include those features and capabilities that we believe are most

useful to you, but we are not absolutely complete in our coverage. Once you

are familiar with a good working set of GLSL, you probably should read the

GLSL specication to see what else is there—especially since the language will

continue to evolve over time.

1

We are indebted to the GLSL Shader Language Specication document

both for the overall information it contains and for its excellent tables of GLSL

functions and operations that we have borrowed from extensively.

Factors that Shape Shader Languages

Shader languages operate in a dierent environment and with dierent goals

than general-purpose languages. Their environment is the processing capabil-

ity of graphics cards, which diers in some important ways from the capability

of a general CPU, and their goals are tightly focused on supporting graphics

operations, rather than more general kinds of computations. These capabilities

shape the language in signicant ways, and it is important that you under-

stand their impacts as you write shaders.

GLSL shader capabilities are very much

a moving target. This chapter and all

our examples are based on GLSL 4.1.

However, we also include many features

that are deprecated in that standard but are

available in compatibility mode, because

they may be helpful to someone learning to

work with shaders for the first time.

In order to keep current on GLSL,

you should consult [32] from time to time.

1

You will not need a new copy of glman,

however, because OpenGL will compile

only the GLSL shaders, but you may need

to get a new OpenGL driver.

1. Good resources: “OpenGL.” Khronos. Available at hp://www.khronos.org/opengl/.

“OpenGL 4.2 API Quick Reference Card.” Khronos. Available at hp://www.khronos.org/les/

opengl42-quick-reference-card.pdf, 2010.

“OpenGL Shading Language.” OpenGL. Available at hp://www.opengl.org/documentation/glsl/,

2011.