Single file geometryshader

geometryshader.cpp

#include "SDL.h"
// OSX only - change on other platforms
#include <OpenGL/gl3.h>

#include <stdlib.h> //rand()
#include <iostream>
#include <string>

static bool quitting = false;
static SDL_Window *window = NULL;
static SDL_GLContext gl_context;

GLuint vertexBuffer, vertexArrayObject, shaderProgram;
GLint positionAttribute, colorAttribute;

void loadBufferData(){
    // vertex position, color
    float vertexData[48] = {
        -1, 0, 0.0, 1.0 ,  0.0, 1.0, 0.0, 1.0  ,
        -1, -1, 0.0, 1.0 ,  1.0, 0.0, 0.0, 1.0  ,
        0,  0, 0.0, 1.0 ,  0.0, 0.0, 1.0, 1.0  ,
        0, -1, 0.0, 1.0 ,  1.0, 1.0, 1.0, 1.0 ,
        1, 0, 0.0, 1.0 ,  1.0, 1.0, 1.0, 1.0 ,
        1, -1, 0.0, 1.0 ,  1.0, 1.0, 1.0, 1.0 ,
    };
    
    glGenVertexArrays(1, &vertexArrayObject);
    glBindVertexArray(vertexArrayObject);
    
    glGenBuffers(1, &vertexBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, 48 * sizeof(float), vertexData, GL_STATIC_DRAW);
    
    glEnableVertexAttribArray(positionAttribute);
    glEnableVertexAttribArray(colorAttribute);
    int vertexSize =sizeof(float)*8;
    glVertexAttribPointer(positionAttribute, 4, GL_FLOAT, GL_FALSE,vertexSize , (const GLvoid *)0);
    glVertexAttribPointer(colorAttribute  , 4, GL_FLOAT, GL_FALSE, vertexSize, (const GLvoid *)(sizeof(float)*4));
}

GLuint initShader(const char* vShader,const char* gShader, const char* fShader, const char* outputAttributeName) {
    struct Shader {
        GLenum       type;
        const char*      source;
    }  shaders[3] = {
        { GL_VERTEX_SHADER, vShader },
        { GL_GEOMETRY_SHADER, gShader},
        { GL_FRAGMENT_SHADER, fShader }
    };
    
    GLuint program = glCreateProgram();
    
    for ( int i = 0; i < 3; ++i ) {
        Shader& s = shaders[i];
        GLuint shader = glCreateShader( s.type );
        glShaderSource( shader, 1, (const GLchar**) &s.source, NULL );
        glCompileShader( shader );
        
        GLint  compiled;
        glGetShaderiv( shader, GL_COMPILE_STATUS, &compiled );
        if ( !compiled ) {
            std::cerr << " failed to compile:" << std::endl;
            GLint  logSize;
            glGetShaderiv( shader, GL_INFO_LOG_LENGTH, &logSize );
            char* logMsg = new char[logSize];
            glGetShaderInfoLog( shader, logSize, NULL, logMsg );
            std::cerr << logMsg << std::endl;
            delete [] logMsg;
            
            exit( EXIT_FAILURE );
        }
        
        glAttachShader( program, shader );
    }
    
    /* Link output */
    glBindFragDataLocation(program, 0, outputAttributeName);
    
    /* link  and error check */
    glLinkProgram(program);
    
    GLint  linked;
    glGetProgramiv( program, GL_LINK_STATUS, &linked );
    if ( !linked ) {
        std::cerr << "Shader program failed to link" << std::endl;
        GLint  logSize;
        glGetProgramiv( program, GL_INFO_LOG_LENGTH, &logSize);
        char* logMsg = new char[logSize];
        glGetProgramInfoLog( program, logSize, NULL, logMsg );
        std::cerr << logMsg << std::endl;
        delete [] logMsg;
        
        exit( EXIT_FAILURE );
    }
    
    /* use program object */
    glUseProgram(program);
    
    return program;
}


void loadShader(){
    const char *  vert = R"(#version 150
    
    in vec4 position;
    
    
    void main (void)
    {
        gl_Position = position;
    })";
    const char *  frag = R"(#version 150
    
    in vec4 fColor;
    out vec4 fragColor;
    
    void main(void)
    {
        fragColor = fColor;
    })";
    
    // Geometry shader
    const GLchar* geom = R"glsl(
#version 150
    
    layout(triangles) in;
    layout(triangle_strip, max_vertices = 3) out;
    
    out vec4 fColor;
    
    const float PI = 3.1415926;
    
    void main()
    {
        for (int i=0;i<3;i++){
            gl_Position = gl_in[i].gl_Position ;
            if (i==0){
                fColor = vec4(1.0,0.0,0.0,1.0);
            } else if (i==1){
                fColor = vec4(0.0,1.0,0.0,1.0);
            } else {
                fColor = vec4(0.0,0.0,1.0,1.0);
            }
            
            EmitVertex();
        }
        
        EndPrimitive();
    }
    )glsl";
    
    shaderProgram = initShader(vert, geom, frag, "fragColor");
    
    colorAttribute = glGetAttribLocation(shaderProgram, "color");
    if (colorAttribute < 0) {
        std::cerr << "Shader did not contain the 'color' attribute." << std::endl;
    }
    positionAttribute = glGetAttribLocation(shaderProgram, "position");
    if (positionAttribute < 0) {
        std::cerr << "Shader did not contain the 'position' attribute." << std::endl;
    }
}

void setup() {
    std::cout << "OpenGL version "<<glGetString(GL_VERSION)<<std::endl;
    loadShader();
    loadBufferData();
    
    glEnable(GL_DEPTH_TEST);
}

void render() {
    
    SDL_GL_MakeCurrent(window, gl_context);
    
    
    glClearColor( 0.0f, 0.0f, 0.0f, 1.0f );
    glClear( GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT );
    
    glUseProgram(shaderProgram);
    
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 6);
    
    SDL_GL_SwapWindow(window);
    
}


int main(int argc, char *argv[]) {
    
    
    if (SDL_Init(SDL_INIT_VIDEO|SDL_INIT_EVENTS) != 0) {
        SDL_Log("Failed to initialize SDL: %s", SDL_GetError());
        return 1;
    }
    // Use a core profile setup.
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    window = SDL_CreateWindow("title", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 512, 512, SDL_WINDOW_OPENGL);
    
    gl_context = SDL_GL_CreateContext(window);
    
    setup();
    
    while(!quitting) {
        
        SDL_Event event;
        while( SDL_PollEvent(&event) ) {
            if(event.type == SDL_QUIT) {
                quitting = true;
            }
        }
        
        render();
        SDL_Delay(2);
        
    }
    
    SDL_GL_DeleteContext(gl_context);
    SDL_DestroyWindow(window);
    SDL_Quit();
    
    exit(0);
    
} //main