Shaders in OpenGL Marshall Hahn

Introduction to Shaders in OpenGL In this talk, the basics of OpenGL Shading Language will be covered. This includes topics such as: vertex shaders, fragment shaders, and how they communicate useful built-in functions and datatypes an example shader that implements bump mapping how to compile, initialize and activate a shader

OpenGL 1.5

OpenGL 2.0

OpenGL Shading Language (GLSL) A C-based language, has similar syntax and flow control a little more restrictive in some areas  Functions: return by value, pass by value Has special built-in variables for input\output. Their names always begin with gl_ Has built-in datatypes for matrices and vectors Useful built-in functions: dot, cos, sin, mix, …

OpenGL Shading Language (GLSL): Swizzling The normal structure-member selector (.) is also used to SWIZZLE components of a vector: select or rearrange components by listing their names after the swizzle operator (.)SWIZZLE vec4 v4; v4.rgba; // is a vec4 and the same as just using v4, v4.rgb; // is a vec3, v4.b; // is a float, v4.xy; // is a vec2, v4.xgba; // is illegal - the component names do not come from // the same set. The component names can be out of order to rearrange the components, or they can be replicated to duplicate the components:

The Three Shader Variable Types Uniform variables: can be changed once per primitive  uniform bool bumpon; Attribute variables: can be changed anytime, they represent attributes that are associated with vertices  attribute vec3 tangentVec; Varying variables: are used communicate between vertex shader and fragment shader. They are automatically interpolated across the polygon.  varying vec3 lightVec;

Normal Maps An image where each pixel represents a normal:  R -> x, G -> y, B -> z Each normal is perturbed a small amount, so normal maps tend to be “bluish” in colour R, G, and B each range from 0.0 to 1.0 N.x = ( R – 0.5 ) * 2.0, …

Normal Mapping using Tangent Space Np is in tangent space, L and V are in WCS So we build a frame at the P using N, T (tangent) and N x T We then transform L and V into this frame

Normal Mapping using Tangent Space Suppose we have two points: S in Tangent Space and O in World Space This is the transformation we need:

Normal Mapping Vertex Shader 1.uniform bool bumpon; 2.attribute vec3 tangentVec; 3.varying vec3 lightVec; 4.varying vec3 eyeVec; 5. 6.void main (void) 7.{ 8. vec4 ecPosition = gl_ModelViewMatrix * gl_Vertex; gl_Position = ftransform(); 11. gl_TexCoord[0] = gl_MultiTexCoord0;//texture mapping stuff vec3 orgLightVec = ( gl_LightSource[0].position.xyz - ecPosition.xyz); vec3 n = normalize(gl_NormalMatrix * gl_Normal); 16. vec3 t = normalize(gl_NormalMatrix * tangentVec); 17. vec3 b = cross(n, t); lightVec.x = dot( orgLightVec, t); 20. lightVec.y = dot( orgLightVec, b); 21. lightVec.z = dot( orgLightVec, n); vec3 position = -ecPosition.xyz; 24. eyeVec.x = dot( position, t); 25. eyeVec.y = dot( position, b); 26. eyeVec.z = dot( position, n); 27.}

Normal Mapping Fragment Shader 1.uniform bool bumpon; 2.uniform sampler2D normalMap;//must initialize with texture unit integer 3.varying vec3 lightVec; 4.varying vec3 eyeVec; 5. 6.void main (void) 7.{ 8. vec3 N = vec3(0.0, 0.0, 1.0); 9. if( bumpon ) N = normalize( ( (texture2D(normalMap, gl_TexCoord[0].xy).xyz) - 0.5) * 2.0 ); vec3 L = normalize(lightVec); 12. vec3 V = normalize(eyeVec); 13. vec3 R = reflect(L, N); float pf = pow( dot(R, V), gl_FrontMaterial.shininess ); vec4 GlobalAmbient = gl_LightModel.ambient; 18. vec4 Ambient = gl_LightSource[0].ambient * gl_FrontMaterial.ambient; 19. vec4 Diffuse = gl_LightSource[0].diffuse * gl_FrontMaterial.diffuse * dot(N, L); 20. vec4 Specular = gl_LightSource[0].specular * gl_FrontMaterial.specular * pf; vec4 color1 = GlobalAmbient + Ambient + Diffuse; 23. vec4 color2 = Specular; gl_FragColor = clamp( color1 + color2, 0.0, 1.0 ); 26.}

GLEE(GL Easy Extension library) GLee (GL Easy Extension library) is a free cross-platform extension loading library for OpenGL It provides seamless support for OpenGL functions up to version 2.1 and over 360 extensions Microsoft’s OpenGL API only supports up to OpenGL 1.1

Compilation char* vertSrc = readShaderToString(vertShader); char* fragSrc = readShaderToString(fragShader); vs = glCreateShader(GL_VERTEX_SHADER); fs = glCreateShader(GL_FRAGMENT_SHADER); glShaderSource(vs, 1, &vertSrc, NULL); glShaderSource(fs, 1, &fragSrc, NULL); glCompileShader(vs); printOpenGLError(); // Check for OpenGL errors glGetShaderiv(vs, GL_COMPILE_STATUS, &vertCompiled); printShaderInfoLog(vs); glCompileShader(fs); printOpenGLError(); // Check for OpenGL errors glGetShaderiv(fs, GL_COMPILE_STATUS, &fragCompiled); printShaderInfoLog(fs); if (!vertCompiled || !fragCompiled) exit(-1);

Compilation // Create a program object and attach the two compiled shaders int program = glCreateProgram(); glAttachShader(program, vs); glAttachShader(program, fs); // Link the program object and print out the info log glLinkProgram(program); printOpenGLError(); // Check for OpenGL errors glGetProgramiv(program, GL_LINK_STATUS, &linked); printProgramInfoLog(); if (!linked) exit(-1)

Initialization and Activation Call glUseProgram( program ) to activate a shader.  The default functionality will be disabled  Can switch between multiple shaders while rendering a frame Initialization example: GLint loc = glGetUniformLocation(program, "bumpon"); glUniform1i( loc, GL_TRUE ); GLint loc = glGetUniformLocation(program, "normalMap"); glUniform1i( loc, 0 ); GLint loc = glGetUniformLocation(program, "tangentVec"); glVertexAttrib3f(loc, v1, v2, v3);

For more information, see the spec ngSpec.Full pdf