Introduction

The OpenGL Shading Language (GLSL) Version 4 brings unprecedented power and flexibility to programmers interested in creating modern, interactive, and graphical programs. It allows us to harness the power of modern Graphics Processing Units (GPUs) in a straightforward way by providing a simple yet powerful language and API. Of course, the first step toward using GLSL is to create a program that utilizes the OpenGL API. GLSL programs don't stand on their own; they must be a part of a larger OpenGL program. In this chapter, we will provide some tips and techniques for getting started. We'll cover how to load, compile, link, and export a GLSL shader program. First, let's start with some background.

GLSL

The GLSL is a fundamental and integral part of the OpenGL API. Every program written using the OpenGL API will internally utilize one or several GLSL programs. These "mini-programs" are referred to as shader programs. A shader program usually consists of several components called shaders. Each shader executes within a different stage of the OpenGL pipeline. Each shader runs on the GPU, and as the name implies, they (typically) implement the algorithms related to lighting and shading effects. However, shaders are capable of doing much more than just shading. They can perform animation, generate additional geometry, tessellate geometry, or even perform generalized computation.

The field of study called General Purpose Computing on Graphics Processing Units (GPGPU) is concerned with the utilization of GPUs (often using specialized APIs such as CUDA or OpenCL) to perform general-purpose computations such as fluid dynamics, molecular dynamics, and cryptography. With compute shaders, introduced in OpenGL 4.3, we can now do GPGPU within OpenGL. See Chapter 11, Using Compute Shaders, for details about using compute shaders.

Shader programs are designed for direct execution on the GPU and are executed in parallel. For example, a fragment shader might be executed once for every pixel, with each execution running simultaneously. The number of processors on the graphics card determines how many can be executed at one time. This makes shader programs incredibly efficient, and provides the programmer with a simple API for implementing highly-parallel computation.

Shader programs form essential parts of the OpenGL pipeline. Prior to OpenGL Version 2.0, the shading algorithm was hardcoded into the pipeline and had only limited configurability. When we wanted to implement custom effects, we used various tricks to force the fixed-function pipeline into being more flexible than it really was. With the advent of GLSL, we now have the ability to replace this hardcoded functionality with our own programs written in GLSL, thus giving us a great deal of additional flexibility and power. For more details on this programmable pipeline, see the introduction to Chapter 3, The Basics of GLSL Shaders.

In fact, OpenGL Version 3.2 and above not only provide this capability, but they require shader programs as part of every OpenGL program. The old fixed-function pipeline has been deprecated in favor of a new programmable pipeline, a key part of which is the shader program written in GLSL.

Profiles – core versus compatibility

OpenGL Version 3.0 introduced a deprecation model, which allowed for the gradual removal of functions from the OpenGL specification. Functions or features can be marked as deprecated, meaning that they are expected to be removed from a future version of OpenGL. For example, immediate mode-rendering using glBegin/glEnd was marked as deprecated in version 3.0 and removed in version 3.1.

In order to maintain backwards compatibility, compatibility profiles were introduced with OpenGL 3.2. A programmer that is writing code for a particular version of OpenGL (with older features removed) would use the core profile. Those who wanted to maintain compatibility with older functionality could use the compatibility profile.

It may be somewhat confusing that there is also the concept of a forward-compatible context, which is distinguished slightly from the concept of a core/compatibility profile. A context that is considered forward-compatible basically indicates that all deprecated functionality has been removed. In other words, if a context is forward-compatible, it only includes functions that are in the core, but not those that were marked as deprecated. Some Windows APIs provide the ability to select a forward-compatible status along with the profile.

The steps for selecting a core or compatibility profile depend on the Windows system's API. For example, with GLFW, one can select a forward-compatible, 4.6 core profile using the following code:

glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 4); 
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 6); 
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE); 
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE); 
 
GLFWwindow *window = glfwCreateWindow(800, 600, "Title", nullptr, nullptr);

All programs in this book are designed to be compatible with a forward-compatible OpenGL 4.6 core profile. However, many of them can be used with older versions or even compatibility profiles.