OpenGL Shading Language

lastmod 2024-04-30created 2014-05-0313 edits

GLSL (OpenGL Shading Language) is a high-level shading language based on the C language. It is also known as GLslang. Similar to HLSL, it was created by the OpenGL ARB (Architecture Review Board) so that developers can directly control the graphics pipeline without using assembly language or hardware-specific languages.

GLSL has profiles, so developers can also directly convert code written in Cg.

Vertex Shader

The Vertex Shader defines where vertices made of 3D coordinates are drawn on the 2D screen. The gl_Position variable is predefined and stores the value pointing to the position of a point on the screen.

void main(void) { 
    const vec4 vertices[3] = 
        vec4[3](vec4( 0.25, -0.25, 0.5, 1.0), 
                vec4(-0.25, -0.25, 0.5, 1.0), 
                vec4( 0.25, 0.25, 0.5, 1.0)); 

    // 화면에 3개의 점을 그린다.
    gl_Position = vertices[gl_VertexID]; 
}

Geometry Shader

• (Learn OpenGL translation) 5-9. Advanced OpenGL - Geometry Shader

Between the vertex shader and the fragment shader there is an optional shader stage called the geometry shader. A geometry shader takes as input a set of vertices that form a single primitive, for example a point or a triangle. The geometry shader can transform these vertices as it sees fit before sending them to the next shader stage. What makes the geometry shader interesting, however, is that it can convert the vertices into completely different primitives, generating more vertices than were originally given.

Example:

#version 330 core
layout (points) in;
layout (line_strip, max_vertices = 2) out;

void main() {    
    gl_Position = gl_in[0].gl_Position + vec4(-0.1, 0.0, 0.0, 0.0); 
    EmitVertex();

    gl_Position = gl_in[0].gl_Position + vec4( 0.1, 0.0, 0.0, 0.0);
    EmitVertex();

    EndPrimitive();
}  

Fragment Shader

The Fragment Shader defines what color each pixel is drawn in. It returns a color buffer, and objects not represented in the color buffer are all rendered black. gl_FragColor is the variable used as the function's return value and defines the color shown on the screen.

#ifdef GL_ES
precision mediump float;
#endif

void main() {
    // Vertex Shader 에 의해 정의된 영역을 빨간색으로 칠한다.
    gl_FragColor = vec4(1.0,0.0,0.0,1.0);
}

Versions

GLSL versions have evolved alongside specific versions of the OpenGL API. It is only with OpenGL versions 3.3 and above that the GLSL and OpenGL major and minor version numbers match. These versions for GLSL and OpenGL are related in the following table:

Favorite site

• OpenGL Shading Language
• Wikipedia (en) GLSL
• Wikipedia (en) OpenGL Shading Language
• Summary of OpenGL shader creation functions
• Stackoverflow: Finding supported GLSL version
• OpenGL ES 2.0 shader pipeline compared with Direct3D
• OpenGL Shader concepts | Hyeonwoo's dev notes

Guide

• GLSL Version Check
• GLSL Shader 5 - Setup/Overview
• Converting YUV420 format colors to RGB using an OpenGL Shader

Tutorial

• GLSL 1.2 Tutorial
• OpenGL Shader tutorial ¹²
• OpenGL Shader tutorial ¹³

References