Question

I’m working on a program with OpenGL/SDL, but the window won’t draw. The window’s entry appears in the taskbar and the alt+tab menu, but no thumbnail is shown. When I click it, it’s marked as active, but nothing changes onscreen.

I can verify that glClear(GL_COLOR_BUFFER_BIT) and SDL_GL_SwapBuffers() are being called from my render loop, and that SDL_SetVideoMode is succeeding, but it still won’t render. It’s not a shared library issue, since other SDL/GL apps work fine.

Any ideas?

EDIT:

Here’s the graphics initialization:

void GraphicsManager::initGraphics() {
    // Access prefs
    PreferencesManager* prefMgr = PreferencesManager::getInstance();
    int width = prefMgr->getIntKey("res_width");
    int height = prefMgr->getIntKey("res_height");

    if(SDL_InitSubSystem(SDL_INIT_VIDEO) != 0) {
        fprintf(stderr, "Cannot initialize SDL video!\n");
        exit(1);
    }

    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);

    m_screen = SDL_SetVideoMode(width, height, 0, SDL_OPENGL);
    if(!m_screen) {
        fprintf(stderr, "Error: Invalid screen pointer\n");
        exit(2);
    }

    SDL_ShowCursor(SDL_DISABLE);

    // Setup OpenGL stuffs
    glDisable(GL_DEPTH_TEST); // We're only using 2D
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glViewport(0.0, 0.0, width, height);
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0, width, height, 0.0, -1.0f, 1.0f);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glEnable(GL_TEXTURE_2D);
    glClear(GL_COLOR_BUFFER_BIT);
    SDL_GL_SwapBuffers();

    // Init FPS stuff
    m_avgFps = 60.0f;
    m_lastUpdate = SDL_GetTicks();
}

Here’s my main loop:

while(stateMgr->getState() != GS_QUIT) {
    // Process input events
    inputMgr->processEvents();

    // Update the current system mode
    stateMgr->update();

    // Update GUI state
    guiMgr->update();

    // Render
    gfxMgr->render();
    //char buf[16];
    //snprintf(buf, 16, "FPS: %4.2f", gfxMgr->getFramerate());
    //testWindow->getChild("TestRoot/Framerate")->setText(buf);
    SDL_Delay(10);
}

And here’s the render function:

void GraphicsManager::render() {
    std::list<IRenderCallback*>::iterator rci;

    // Clear the screen
    glClear(GL_COLOR_BUFFER_BIT);

    // Run render callbacks
    /*for(rci=m_renderCallbacks.begin();rci != m_renderCallbacks.end();rci++) {
        (*rci)->preRender();
    }

    // Iterate through all renderables and render them
    std::list<Renderable*>::iterator i;
    for(i=m_renderables.begin();i != m_renderables.end();i++) {
        (*i)->render();
    }

    // Run render callbacks
    for(rci=m_renderCallbacks.begin();rci != m_renderCallbacks.end();rci++) {
        (*rci)->preFlip();
    }*/

    // Flip the buffers
    SDL_GL_SwapBuffers();

    // Update FPS stuff
    Uint32 now = SDL_GetTicks();
    Uint32 timeTaken = now - m_lastUpdate;
    m_lastUpdate = now;
    double seconds = (double)timeTaken / 1000.0f;
    m_avgFps = (1.0f / seconds);

    // Run render callbacks
    for(rci=m_renderCallbacks.begin();rci != m_renderCallbacks.end();rci++) {
        (*rci)->postRender();
    }
}

Answer 1

Break it down. Do the (mostly-ish) simplest thing that should work:

#include <SDL.h>
#include <SDL_opengl.h>
#include <cstdio>
#include <iostream>

using namespace std;

int main( int argc, char** argv )
{
    if(SDL_InitSubSystem(SDL_INIT_VIDEO) != 0) {
        fprintf(stderr, "Cannot initialize SDL video!\n");
        exit(1);
    }

    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
    SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);

    int width = 640;
    int height = 480;

    SDL_Surface* m_screen = SDL_SetVideoMode(width, height, 0, SDL_OPENGL);
    if(!m_screen) {
        fprintf(stderr, "Error: Invalid screen pointer\n");
        exit(2);
    }

    cout << "GL_VERSION  : " << glGetString(GL_VERSION) << endl;
    cout << "GL_VENDOR   : " << glGetString(GL_VENDOR) << endl;
    cout << "GL_RENDERER : " << glGetString(GL_RENDERER) << endl;

    SDL_ShowCursor(SDL_DISABLE);

    // Setup OpenGL stuffs
    glDisable(GL_DEPTH_TEST); // We're only using 2D
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glViewport(0.0, 0.0, width, height);
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0, width, height, 0.0, -1.0f, 1.0f);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    //glEnable(GL_TEXTURE_2D);
    glClear(GL_COLOR_BUFFER_BIT);
    SDL_GL_SwapBuffers();

    float angle = 0;
    bool running = true;
    while( running )
    {
        // handle all pending events
        SDL_Event event;
        while( SDL_PollEvent(&event) )
        {
            switch (event.type)
            {
            case SDL_QUIT:  
                running = false; 
                break;
            default:        
                break;
            }
        }

        angle += 1;
        while( angle > 360 )
            angle -= 360;

        // Render
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glPushMatrix();
        glTranslatef( width/2, height/2, 0 );
        glScalef( 150, 150, 0 );

        glRotatef( angle, 0, 0, 1 );
        glColor3ub(255,0,0);
        glBegin(GL_TRIANGLES);
        glVertex2i(0,0);
        glVertex2i(1,0);
        glVertex2i(1,1);
        glEnd();

        glPopMatrix();

        // Flip the buffers
        SDL_GL_SwapBuffers();

        SDL_Delay(10);
    }

    SDL_Quit();
    return 0;
}

If that works all you have to do (ha!) is figure out how the simple program execution flow differs from your large program. If it doesn’t work your environment is probably goofed somehow.

What do the GL_VERSION/VENDOR/RENDERER checks print on your system?