OpenGl glViewport 调用重置 vao、vbo 和着色器程序

Question

So I've been starting with OpenGL by following this tutorial and managed to get everything to draw fine, but when I resize my viewport, the vao, vbo and shader program are unbound. I need to rebind everything and resend the buffer data , I don't know if this is the expected behaviour, but it seems a bit ridiculous to me so maybe I'm missing something. I did some searching and tried couple of different ways (with/without vaos, using glVertexAttribFormat / glVertexAttribBinding...) with no progress.

I made a logGl functions to get as much information as I could:

void logGl() {
    int value, value2;
    glGetIntegerv(GL_ARRAY_BUFFER_BINDING, &value);
    std::cout << "bound buffer: " << value << std::endl;

    glGetIntegerv(GL_VERTEX_ARRAY_BINDING, &value);
    std::cout << "bound vao: " << value << std::endl;

    glGetIntegerv(GL_CURRENT_PROGRAM, &value);
    std::cout << "bound program: " << value << std::endl;

    glGetIntegerv(GL_MAJOR_VERSION, &value);
    glGetIntegerv(GL_MINOR_VERSION, &value2);
    std::cout << "GL version: " << value << "." << value2 << std::endl;

    float data[9];
    glGetBufferSubData(GL_ARRAY_BUFFER, 0, 9 * sizeof(float), data);
    std::cout << "Buffer Data: ";
    for (size_t i=0; i<9; i++) { std::cout << data[i] << " ";};
    std::cout << std::endl;
}

this is the output:

Shader Program ID: 3
bound buffer: 1
bound vao: 1
bound program: 3
GL version: 4.6
Buffer Data: -0.5 -0.5 0 0 0.5 0 0.5 -0.5 0 
changing viewport ******* 
bound buffer: 0
bound vao: 0
bound program: 0
GL version: 4.6
Buffer Data: 1.23555e-31 4.59163e-41 28.875 0 30.0201 0 2.05719e+32 4.55772e-41 1.23552e-31 
We see everything is unbound (Here I recreate shader program)
Shader Program ID: 39
Viewport size: 0 0 801 600
In main loop, just before isssuing draw call:
bound buffer: 1
bound vao: 0
bound program: 39
GL version: 4.6
Buffer Data: 3.8357e+34 4.55772e-41 3.47159e+34 4.55772e-41 4.56443e+34 4.55772e-41 1.5695e+11 3.09e-41 1.23552e-31 
Error: 1282
--------------------------------

The error code 1282 did not really provide me with any clue, I did not get any compiling / linking errors from my setShaders function.

This is my main loop, I can provide the implementations of the shader function if needed, but it's pretty much tutorial textbook..

int main() {
    // Init SDL, get GL context, set initial viewport init glew ...
    Systems sys = initWindow(800, 600);

    // Init a buffer
    float vertices[] = {
            -0.5f, -0.5f, 0.0f,
            0.0f, 0.5f, 0.0f,
            0.5f, -0.5f, 0.0f
    };

    unsigned int vbo, vao;
    glGenVertexArrays(1, &vao);
    glBindVertexArray(vao);   // bind first!

    glGenBuffers(1, &vbo);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3*sizeof(float), (void *) 0);
    glEnableVertexAttribArray(0);

    unsigned int shader = setShaders("Shaders.txt");

    logGl();

    bool running = true;
    while (running) {
        bool log = false;

        SDL_Event event;
        while (SDL_PollEvent(&event)) {
            switch (event.type) {
                case SDL_QUIT:
                    running = false;

                case SDL_KEYDOWN:
                    if (event.key.keysym.sym == SDLK_ESCAPE) running = false;

                case SDL_WINDOWEVENT:
                    switch (event.window.event) {
                        case SDL_WINDOWEVENT_RESIZED:
                            int dims[4];
                            std::cout << "changing viewport ******* \n";
                            glViewport(0, 0, event.window.data1, event.window.data2);
//                            glViewport(0, 0, 500, 500);
                            logGl();
                            std::cout << "We see everything is unbound" << std::endl;

                            // Updating viewport unbinds vao, shader program
                            // and seems to empty vbo data
                            // since I get this error code after viewport call at glDraw:

                            shader = setShaders("Shaders.txt");
                            glUseProgram(shader);

                            glGetIntegerv(GL_VIEWPORT, dims);
                            std::cout << "Viewport size: "
                                      << dims[0] << " "
                                      << dims[1] << " "
                                      << dims[2] << " "
                                      << dims[3] << std::endl;

                            log = true;
                    }

            }
        }

        // random bg color
        Uint32 rdCol = SDL_GetPerformanceCounter();
        glClearColor((float) (rdCol & 0x0000FF00) / 0x0000FF00,
                     (float) (rdCol & 0x00FF0000) / 0x00FF0000,
                     (float) (rdCol & 0xFF000000) / 0xFF000000, 1);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        // Why is this not needed ..???
        glBindVertexArray(vao);

        ///////////////////////////////////////////////////////////////////////
        // *** The four lines below are necessary to keep drawing on win resize
        glBindBuffer(GL_ARRAY_BUFFER, vbo);
//        glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
//
//        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void *) 0);
//        glEnableVertexAttribArray(0);
        ///////////////////////////////////////////////////////////////////////


        if (log) {
            std::cout << "In main loop, just before isssuing draw call:" << std::endl;
            logGl();
            std::cout << "Error: " << glGetError() << std::endl;
            std::cout << "--------------------------------" << std::endl;
        }
        glDrawArrays(GL_TRIANGLES, 0, 3);

        /* 'flip' Window */
        SDL_GL_SwapWindow(sys.window);

    }

    SDL_FreeSurface(sys.winSurf);
    SDL_DestroyWindow(sys.window);
    SDL_Quit();

    return 0;
}

So the big question is why eveything gets unbound when I call glViewport, as I would believe it should keep drawing only by binding vao and vbo (which works until I resize the window)

Answer 1

I have found the culprit, which is that when resizing the window in SDL, it creates a new OpenGL context and thus removes every binding made. So, I could either rebind everything (ridiculous but it works) or use GLFW, SFML or other librairies. I found my answer here , if anyone ever encounters this problem.