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My voxel renderer app fails to create a framebuffer object only during launch / initial window resize event. It is an ERROR: GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT error. I'm working with the latest SDL version (building from the main branch), and I think that the initial render/launch of my app fails because it cannot create an OpenGL 3.0 framebuffer object until certain events occur like SDL_EVENT_WINDOW_RESIZED. The reason I think this is because once I move the window manually on screen during runtime the FBO is created without errors. The problem is that I don't want to make users have to manually reposition the window to render the voxel scene.

I create the FBO, RBO, and texture in a C++ lambda like this:

    auto create_fbo = [](GLuint width, GLuint height)->tuple<GLuint, GLuint, GLuint> {
        GLuint fbo, depthRenderbuffer, texture;
        glGenFramebuffers(1, &fbo);
        glGenRenderbuffers(1, &depthRenderbuffer);
        glGenTextures(1, &texture);
        glBindFramebuffer(GL_FRAMEBUFFER, fbo);
        glBindTexture(GL_TEXTURE_2D, texture);
        glActiveTexture(GL_TEXTURE6);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texture, 0);
        glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
        glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH24_STENCIL8, width, height);
        glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);

        // Check for FBO initialization errors
        GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
        if (status != GL_FRAMEBUFFER_COMPLETE) {
            switch (status) {
                case GL_FRAMEBUFFER_UNDEFINED:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_UNDEFINED\n");
                    break;
                case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT\n");
                    break;
                case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT\n");
                    break;
                case GL_FRAMEBUFFER_UNSUPPORTED:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_UNSUPPORTED\n");
                    break;
                case GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE\n");
                    break;
#if !defined(__EMSCRIPTEN__)
                case GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER\n");
                    break;
                case GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER\n");
                    break;
#endif
                default:
                    SDL_LogError(SDL_LOG_CATEGORY_ERROR, "Unknown FBO error\n");
                    break;
            }
            return {};
        }

    SDL_Log("Creating FBO with width: %d and height: %d\n", width, height);

        glBindFramebuffer(GL_FRAMEBUFFER, 0);
        return {fbo, depthRenderbuffer, texture};
    };

I call this function on SDL window resize events (which are handle in an SDL_PollEvent loop):

case SDL_EVENT_WINDOW_EXPOSED:
case SDL_EVENT_WINDOW_RESIZED: {
     if (glIsTexture(get<2>(fbo_tuple))) {
        glDeleteTextures(1, &get<2>(fbo_tuple));
        glDeleteRenderbuffers(1, &get<1>(fbo_tuple));
        glDeleteFramebuffers(1, &get<0>(fbo_tuple));
}
    fbo_tuple = create_fbo(voxel_scene_w, voxel_scene_h);
}         


break;
            }

The FBO is used with get<0>(fbo_tuple) and it will render as expected, but to get the render to function properly I have to manually trigger the SDL_EVENT_WINDOW_RESIZED or SDL_EVENT_WINDOW_EXPOSED event. It throws the OpenGL error on launch / first resize event.

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2 Answers 2

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One important aspect of FBOs is that the framebuffer and texture is easy to accidentally loop if you have any glBindTexture(...,tex) calls without glBindTexture(...,0) after rendering is done. Basically OpenGL state is leaking from one use to another and it breaks the FBOs. Took like hours and hours of work to figure out why FBOs fail, but leaking the glBindTexture() is good candidate for failing FBOs. Happily WebGL in Emscripten is gives good error messages about "framebuffer and active texture looping".

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I was able to solve this issue by forcing a call on SDL launch (after GL context creation) to create an OpenGL buffer for the voxel scene. So, I think there was no scene being rendered to the FBO on first launch.

I did this code during first resize event on launch:

GLuint buffer;
glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData(GL_ARRAY_BUFFER, size, data, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
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