上下文创建失败

Question

我正在学习 OpenGL，所以我正在尝试绘制 2D 图像。 我首先将所有内容都放在我的 C++ 程序的 main 函数中。效果很好（绘制了 2 个三角形）。

我决定通过制作单独的类来使程序更清晰，... 但现在它不再工作了。当我想制作 OpenGL 上下文时，它失败了。当我显示错误时，我得到：

Failed creating OpenGL context at version requested

在我正在阅读的教程中，他们说这个错误很可能是由于您的显卡不支持 OpenGL 的版本，但如果是这种情况，当我把所有主程序中的代码。

这是出错的部分（上下文创建）：

bool OpenGL_Scene::initializeWindow() {
    // Initialize the SDL
    if(SDL_Init(SDL_INIT_VIDEO) < 0)
    {
        cout << "Error while initializing the SDL : " << SDL_GetError() << endl;
        SDL_Quit();

        return false;
    }

    // Configure OpenGL

    // Use version 3.1
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3); // OpenGL 3.x
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1); // OpenGL x.1 --> OpenGL 3.1

    // Double buffering
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); // Use double buffering (0 to not use it)
    SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);  // Buffer has a depth of 24 bits

    // Make the window
    this->window = SDL_CreateWindow(this->windowTitle.c_str(), SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, this->windowWidth, this->windowHeight, SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL); // SDL_WINDOW_OPENGL necessary to specify that the window will have an OpenGL context attached to it.

    if(this->window == 0) // Initialization failed
    {
        cout << "Error while creating the window : " << SDL_GetError() << endl;
        SDL_Quit();

        return false;
    }

    // Make the OpenGL context given the SDL window
    this->OpenGL_Context = SDL_GL_CreateContext(this->window);

    // Make sure the creation of the context succeeded. If not the problem is probably that the version of OpenGL isn't supported by the graphics card.
    if(this->OpenGL_Context == 0)
    {
        cout << "Could not create the OpenGL context : " << SDL_GetError() << endl;

        SDL_DestroyWindow(window);
        SDL_Quit();
        return false;
    }

    return true;
};

在最后几行中显示了错误，因此程序导致：

Could not create the OpenGL context : Failed creating OpenGL context at version requested

我搜索了很多以找到解决方案并找到了这个： SO topic

所以在上面的代码中我尝试了：

// Configure OpenGL

// Use version 3.1
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3); // OpenGL 3.x
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1); // OpenGL x.1 --> OpenGL 3.1

// Double buffering
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);      // ADDING THIS AVOIDS THAT THE CONTEXT COULD NOT BE CREATED BUT THEN WHEN WE DRAW SOMETHING WE DON'T SEE ANYTHING

SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); // Use double buffering (0 to not use it)
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);  // Buffer has a depth of 24 bits

现在当我用这个额外的行运行程序时，创建了上下文（没有错误，所以OpenGL_Context != 0）但什么都没有绘制。我省略了我正在绘制的代码部分，因为它之前工作过并且我没有更改任何内容。

有人知道可能是什么问题吗？

PS：我正在使用 Macbook Pro (OS X Yosemite (10.10.4))，我的显卡是 NVIDIA GeForce GT 650M 1024 MB

编辑：我尝试调试绘制三角形的代码，但我真的没有看到错误（主要是因为我真的是 OpenGL 的新手）。在下方您可以找到绘图代码。请注意，我不再初始化 GLEW，因为它不是必需的（根据 cmets）。

void OpenGL_Scene::mainLoop() {
    bool end = false;

    // Make vertices (punten) in a table
    // !!! WARNING : Use 1 table for ALL vertices !!! Don't use a separate table for each of the forms, this would slow down the program because you have to send each of the tables to OpenGL !!!

    float vertices[] = {-0.5, -0.5,   0.0, 0.5,   0.5, -0.5,      // 3 Points for first triangle --> (-0.5, -0.5) , (0.0, 0.5) and (0.5, -0.5)  (All in (x, y) --> 2D)
                    -0.8, -0.8,   -0.3, -0.8,   -0.8, -0.3};  // 3 Points for second triangle

    // Before we start drawing, clear the screen
    glClear(GL_COLOR_BUFFER_BIT);

    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, vertices);
    glEnableVertexAttribArray(0); // Activate the table we passed to OpenGL using the identifier (index) passed to OpenGL (in this case 0)

    // Now that OpenGL knows which vertices it has to display we are going to specify what it has to do with these vertices
    glDrawArrays(GL_TRIANGLES, 0, 6); // Draw both triangles

    glDisableVertexAttribArray(0);   // Because it isn't necessary anymore
    SDL_GL_SwapWindow(this->window); // Refresh the screen

    while(!end) {
        // Listen to events and play with them
        SDL_WaitEvent(&this->events); // Will wait for an event and assign it to "events" variable

        if(this->events.window.event == SDL_WINDOWEVENT_HIDDEN)
            cout << "The user has hidden the window !" << endl;
        else if(this->events.window.event == SDL_WINDOWEVENT_CLOSE) {
            cout << "The user closed the window !" << endl;

            glClear(GL_COLOR_BUFFER_BIT); // Clear the screen
            SDL_GL_SwapWindow(this->window);    // Refresh the window

            end = true;
        }  
    }
    // Quitting the SDL and OpenGL properly is done by the destructor
}

所以现在当我执行项目时，首先我创建窗口（这是成功的），然后我调用mainLoop 过程（从上面）。 我使用调试器逐步完成绘图代码，所有程序都被调用（glClear，...），但窗口中没有出现任何内容（它保持黑色）。

Answer 1

我发现了错误。我在初始化 SDL 后正在初始化 OpenGL，这就是为什么如果不强制进入“核心配置文件”就无法创建上下文的原因。

“initializeWindow”中的正确顺序是：

// Configure OpenGL

// Use version 3.1
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3); // OpenGL 3.x
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1); // OpenGL x.1 --> OpenGL 3.1

// Double buffering
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); // Use double buffering (0 to not use it)
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);  // Buffer has a depth of 24 bits

// Initialize the SDL
if(SDL_Init(SDL_INIT_VIDEO) < 0)
{
    cout << "Error while initializing the SDL : " << SDL_GetError() << endl;
    SDL_Quit();

    return false;
}

这样就成功创建了上下文。