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OpenGl Shaders

OpenGl Shaders. http://www.lighthouse3d.com/tutorials/glsl-tutorial/ 2011 Lighthouse3d.com. What?. GLSL 1.2 - GL Shading Language Cg - Nvidia’s shading language ARB extensions (Architecture Review Board) OpenGL 2.0. PipeLINE overview. Visual Pipeline. Fixed function pipeline.

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OpenGl Shaders

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  1. OpenGl Shaders • http://www.lighthouse3d.com/tutorials/glsl-tutorial/ • 2011 Lighthouse3d.com

  2. What? • GLSL 1.2 - GL Shading Language • Cg - Nvidia’s shading language • ARB extensions (Architecture Review Board) • OpenGL 2.0

  3. PipeLINE overview

  4. Visual Pipeline

  5. Fixed function pipeline

  6. Fixed v. Programmable • standard OpenGL: Fixed-function pipeline • add more user control & flexibility: programmable • Pipeline processing - 2 stages • vertex processors • fragment processors

  7. Vertex processor • vertex shader executed once for each vertex • vertex position transformation usually using the modelview and projection matrices • normal transformation, normalization • texture coordinate generation and transformation • lighting per vertex • color computation

  8. Fragment processor • fragment - per pixel data • fragment shader executed once for each fragment • computing colors and texture coordinates per pixel • texture application • fog computation • computing normals for lighting per pixel • can discard the fragment or compute color

  9. Setup for GLSL • Each shader like a C module • compiled separately • linked to OpenGL program • example brick shader: ptgmedia.pearsoncmg.com/images/0321334892/samplechapter/rost_ch06.pdf

  10. Process Overview

  11. Creating SHADERs

  12. Process Overview void glShaderSource(GLuint shader, int numOfStrings, const char **strings, int *lenOfStrings); • shader – the handler to the shader. • numOfStrings – the number of strings in the array. • strings – the array of strings. • lenOfStrings – an array with the length of each string, or NULL, meaning that the strings are NULL terminated.

  13. Incorporating shaders

  14. void setShaders() { char *vs,*fs; v = glCreateShader(GL_VERTEX_SHADER); f = glCreateShader(GL_FRAGMENT_SHADER); vs = textFileRead("toon.vert"); fs = textFileRead("toon.frag"); const char * vv = vs; const char * ff = fs; glShaderSource(v, 1, &vv,NULL); glShaderSource(f, 1, &ff,NULL); free(vs);free(fs); glCompileShader(v); glCompileShader(f); p = glCreateProgram(); glAttachShader(p,v); glAttachShader(p,f); glLinkProgram(p); glUseProgram(p); }

  15. Debugging Is hard - no printf - functions to test compilation & linking e.g. void glGetShaderiv(GLuint object, GLenum type, int *param); Can fetch an ‘infoLog’ to get more about errors

  16. GLSL Variables Read-only in shader value set by program Uniform defined for a primitive (outside glBegin-glEnd) not on a per Vertex basis Attribute on a per Vertex basis - for vertex shaders

  17. GLSL Variables bool float int vec{2,3,4} bvec{2,3,4} ivec{2,3,4} const attribute uniform varying mat2 mat3 mat4 sampler{1D,2D,3D,Cube,1DShadow,2DShadow}

  18. GLSL STATEMENTS if - else for loop continue break discard while do-while each shader - one main() declare in / out parameters

  19. Brick VERTEX Shader • uniform vec3 LightPosition; • const float SpecularContribution = 0.3; • const float DiffuseContribution = 1.0 - SpecularContribution; • varying float LightIntensity; • varying vec2 MCposition; • void main() { • vec3 ecPosition = vec3(gl_ModelViewMatrix * gl_Vertex); • vec3 tnorm = normalize(gl_NormalMatrix * gl_Normal); • vec3 lightVec = = normalize(LightPosition - ecPosition); • vec3 reflectVec = reflect(-lightVec, tnorm); • vec3 viewVec = normalize(-ecPosition); • float diffuse = max(dot(lightVec, tnorm), 0.0); • float spec = 0.0; • if (diffuse > 0.0) { • spec = max(dot(reflectVec, viewVec), 0.0); • spec = pow(spec, 16.0); • } • LightIntensity = DiffuseContribution * diffuse + SpecularContribution * spec; • MCposition = gl_Position = gl_Vertex.xy; ftransform(); • }

  20. Brick Fragment Shader uniform vec3 BrickColor, MortarColor; uniform vec2 BrickSize; uniform vec2 BrickPct; varying vec2 MCposition; varying float LightIntensity; void main() { vec3 color; vec2 position, useBrick; position = MCposition / BrickSize; if (fract(position.y * 0.5) > 0.5) position.x += 0.5; position = fract(position); useBrick = step(position, BrickPct); color = mix(MortarColor, BrickColor, useBrick.x * useBrick.y); color *= LightIntensity; gl_FragColor = vec4(color, 1.0); }

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