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Thread: Does this run properly for anyone else?

  1. #1
    Newbie Newbie
    Join Date
    Jun 2017

    Angry Does this run properly for anyone else?

    Hi everyone. This is my first OpenGL program, and I understand all of of the concepts (VAOs, VBOs, etc...) but just cannot get it to work.

    Code :
    //  main.cpp
    //  opengl_rect_w_shader_header
    //  Created by Pramod on 21/05/2017.
    //  Copyright  2017 Pramod. All rights reserved.
    #include <iostream>
    #include <SDL2/SDL.h>
    #include <OpenGL/gl3.h>
    #include <OpenGL/gl3ext.h>
    SDL_GLContext context;
    SDL_Window * window;
    bool initWindow(){
        if (SDL_Init(SDL_INIT_VIDEO)<0){
            std::cout << "Can't make SDL" << std::endl;
        //what version we are using
        SDL_GL_SetAttribute (SDL_GL_CONTEXT_MAJOR_VERSION, 3);
        //SDL_GL_SetAttribute(SDL_GL_ACCELERATED_VISUAL, 1);
        window = SDL_CreateWindow("Rectangle", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 512, 512, SDL_WINDOW_OPENGL);
        if (!window){
            std::cout << "No window created" << std::endl;
            //window = NULL;
            return false;
        context = SDL_GL_CreateContext(window);
        if (!context){
            std::cout << "No context created" << std::endl;
            return false;
        //double buffering - 24bit z buffer (might need changing for different systems)
        SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
        //Makes buffer swap sync with VBLANK
        glViewport(0, 0, 512, 512);
        std:: cout << "Initialised" << std::endl;
        return true;
    int main(int argc, const char * argv[]) {
        const char * vshaderFile = "vertex.txt";
        const char * fshaderFile = "fragment.txt";
        long length;
        char * vertexBuf;
        FILE * vertexFile = fopen(vshaderFile, "r");
        if(!vertexFile) { std::cout << "Could not open vertex shader" << std::endl; }
        fseek(vertexFile, 0, SEEK_END);
        length = ftell(vertexFile);
        vertexBuf = new char [length + 1];
        fseek(vertexFile, 0, SEEK_SET);
        fread(vertexBuf, length, 1, vertexFile);
        vertexBuf[length] = 0; //makes the terminating character null
        //read fragment shader
        long length2;
        char * fragmentBuf;
        FILE * fragmentFile = fopen(fshaderFile, "r");
        if(!fragmentFile){ std::cout << "Could not open fragment shader" << std::endl; }
        fseek(fragmentFile, 0, SEEK_END);
        length2 = ftell(fragmentFile);
        fragmentBuf = new char [length2 + 1];
        fseek(fragmentFile, 0, SEEK_SET);
        fread(fragmentBuf, length2, 1, fragmentFile);
        fragmentBuf[length2] = 0;
        //makes the source pointers the buffers
        const char *vShaderSource = "#version 330 core\n"
        "layout (location = 0) in vec3 aPos;\n"
        "void main()\n"
        "   gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
        const char *fShaderSource = "#version 330 core\n"
        "out vec4 FragColor;\n"
        "void main()\n"
        "   FragColor = vec4(1.0f, 1.0f, 1.0f, 1.0f);\n"
        //compiles shaders
        GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
        //std::cout << glGetError() << std::endl;
        glShaderSource(fragmentShader, 1, &fShaderSource, NULL);
        int success;
        char infoLog[512];
        glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
        if (!success)
            glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
        GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
        glShaderSource(vertexShader, 1, &vShaderSource, NULL);
        glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
        if (!success)
            glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
        //releases buffers
        //delete[] fragmentBuf;
        //delete[] vertexBuf;
        //fragmentBuf = NULL;
        //vertexBuf = NULL;
        //links program
        GLuint shaderProgram = glCreateProgram();
        glAttachShader(shaderProgram, vertexShader);
        glAttachShader(shaderProgram, fragmentShader);
        glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
        if (!success) {
            glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
        //deletes unneeded shaders
        std:: cout << "Shaders complete" << std:: endl;
        //std::cout << vShaderSource << std::endl << std::endl << fShaderSource << std::endl;
        GLfloat vertices[3][4] = {
            0.5f,  0.5f, 0.0f,  // top right
            0.5f, -0.5f, 0.0f,  // bottom right
            -0.5f, -0.5f, 0.0f,  // bottom left
            -0.5f,  0.5f, 0.0f   // top left
        GLint indices[3][2] = {  // note that we start from 0!
            0, 1, 3,  // first Triangle
            1, 2, 3   // second Triangle
        GLuint VBO, VAO, EBO;
        glGenVertexArrays(1, &VAO);
        glGenBuffers(1, &VBO);
        glGenBuffers(1, &EBO);
        // bind the Vertex Array Object first, then bind and set vertex buffer(s), and then configure vertex attributes(s).
        glBindBuffer(GL_ARRAY_BUFFER, VBO);
        glBufferData(GL_ARRAY_BUFFER, 12 * sizeof (float), vertices, GL_STATIC_DRAW);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), 0);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, 6 * sizeof(int), indices, GL_STATIC_DRAW);
        // note that this is allowed, the call to glVertexAttribPointer registered VBO as the vertex attribute's bound vertex buffer object so afterwards we can safely unbind
        glBindBuffer(GL_ARRAY_BUFFER, 0);
        // remember: do NOT unbind the EBO while a VAO is active as the bound element buffer object IS stored in the VAO; keep the EBO bound.
        //glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
        // You can unbind the VAO afterwards so other VAO calls won't accidentally modify this VAO, but this rarely happens. Modifying other
        // VAOs requires a call to glBindVertexArray anyways so we generally don't unbind VAOs (nor VBOs) when it's not directly necessary.
        bool loop = true;
        glClearColor(0.5f, 0.1f, 0.3f, 1.0f);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_BYTE, (void*)0);
        std::cout << glGetError() << std::endl;
        //std::cout << glCheckError() << std::endl;
        while (loop) {
            SDL_Event e;
                if (e.type == SDL_QUIT) loop = false;
                if (e.type == SDL_KEYDOWN){
                        case SDLK_ESCAPE:
                            loop = false;
                        case SDLK_y:
                            std::cout << glGetString(GL_VERSION) << std::endl;
                    std::cout << "Error: " << SDL_GetError() << std::endl;
                    std::cout << glGetError() << std::endl;

    I have spent at least 3 months looking at this and wondering why it doesn't work...

    What happens when you run the program: Screen goes fuschia purple colour, but does not draw rectangle. I set the rectangle colour to white just to make sure it stood out against the background.
    I made sure that I swap the buffer so that I can see the rectangle. I also checked every single function call against the prototypes and tutorials to make sure I had the right parameters.

    GL version returned is 3.3, and error code is 0. SDL also returns no errors.

    I can't see what I have done wrong compared to the tutorial!Getting-st...Hello-Triangle ... Can someone else run this on their computer and see if the rectangle is drawn or not? (The code doesn't have the #ifdef stuff to make it work on both Windows and Mac/Linux, sorry). Thank you.

    edit: FIXED! I changed GL_UNSIGNED_BYTE in the glDrawElements to GL_UNSIGNED_INT. Phew! Thank you everyone! My first rectangle (well, square...) at last....

  2. #2
    Junior Member Regular Contributor
    Join Date
    Dec 2016
    Ocean Shores, WA USA
    Could it be that your indices array is GLint, but your calling glDrawElements with GL_UNSIGNED_BYTE? Do you need to make them GLuint?


  3. #3
    Junior Member Regular Contributor
    Join Date
    Dec 2016
    Ocean Shores, WA USA
    Yay. Awesome dude.

    Glad to be of service.


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