在Windows上创建并行的屏幕外OpenGL上下文

我正在尝试设置并行多GPU屏幕外渲染上下文。我使用"OpenGL Insights"书，第27章，"NVIDIA Quadro上的多gpu渲染"，我也查看了wglCreateAffinityDCNV文档，但仍然无法确定。

我的机器有2个NVidia Quadro 4000卡(无SLI)。运行在Windows 7 64位。我的工作流程如下:

1. 使用GLFW创建默认的窗口上下文
2. 映射GPU设备
3. 销毁默认的GLFW上下文。
4. 为每个设备创建新的GL上下文(当前只尝试一个)
5. 为每个上下文设置boost线程并使其在该线程中当前。
6. 在每个线程上分别运行渲染过程。(无资源共享)

一切都没有错误地创建并运行，但是一旦我尝试从屏幕外FBO读取像素，我在这里得到一个空指针:

GLubyte* ptr  = (GLubyte*)glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);

同时glError返回"UNKNOWN ERROR"

我认为可能是多线程是问题，但相同的设置在单线程上运行时给出相同的结果。所以我认为这与语境的创造有关。

我是这样做的:

  ////Creating default window with GLFW here .
      .....
         .....

创建屏幕外上下文:

  PIXELFORMATDESCRIPTOR pfd =
{
    sizeof(PIXELFORMATDESCRIPTOR),
    1,
    PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER,    //Flags
    PFD_TYPE_RGBA,            //The kind of framebuffer. RGBA or palette.
    24,                        //Colordepth of the framebuffer.
    0, 0, 0, 0, 0, 0,
    0,
    0,
    0,
    0, 0, 0, 0,
    24,                        //Number of bits for the depthbuffer
    8,                        //Number of bits for the stencilbuffer
    0,                        //Number of Aux buffers in the framebuffer.
    PFD_MAIN_PLANE,
    0,
    0, 0, 0
};
void  glMultiContext::renderingContext::createGPUContext(GPUEnum gpuIndex){
    int    pf;
    HGPUNV hGPU[MAX_GPU];
    HGPUNV GpuMask[MAX_GPU];
    UINT displayDeviceIdx;
    GPU_DEVICE gpuDevice;
    bool bDisplay, bPrimary;
    // Get a list of the first MAX_GPU GPUs in the system
    if ((gpuIndex < MAX_GPU) && wglEnumGpusNV(gpuIndex, &hGPU[gpuIndex])) {
        printf("Device# %d:n", gpuIndex);
        // Now get the detailed information about this device:
        // how many displays it's attached to
        displayDeviceIdx = 0;
        if(wglEnumGpuDevicesNV(hGPU[gpuIndex], displayDeviceIdx, &gpuDevice))
        {   
            bPrimary |= (gpuDevice.Flags & DISPLAY_DEVICE_PRIMARY_DEVICE) != 0;
            printf(" Display# %d:n", displayDeviceIdx);
            printf("  Name: %sn",   gpuDevice.DeviceName);
            printf("  String: %sn", gpuDevice.DeviceString);
            if(gpuDevice.Flags & DISPLAY_DEVICE_ATTACHED_TO_DESKTOP)
            {
                printf("  Attached to the desktop: LEFT=%d, RIGHT=%d, TOP=%d, BOTTOM=%dn",
                    gpuDevice.rcVirtualScreen.left, gpuDevice.rcVirtualScreen.right, gpuDevice.rcVirtualScreen.top, gpuDevice.rcVirtualScreen.bottom);
            }
            else
            {
                printf("  Not attached to the desktopn");
            }
            // See if it's the primary GPU
            if(gpuDevice.Flags & DISPLAY_DEVICE_PRIMARY_DEVICE)
            {
                printf("  This is the PRIMARY Display Devicen");
            }

        }
        ///=======================   CREATE a CONTEXT HERE 
        GpuMask[0] = hGPU[gpuIndex];
        GpuMask[1] = NULL;
        _affDC = wglCreateAffinityDCNV(GpuMask);
        if(!_affDC)
        {
            printf( "wglCreateAffinityDCNV failed");                  
        }
    }
    printf("GPU context created");
}
glMultiContext::renderingContext *
    glMultiContext::createRenderingContext(GPUEnum gpuIndex)
{
    glMultiContext::renderingContext *rc;
    rc = new renderingContext(gpuIndex);
    _pixelFormat = ChoosePixelFormat(rc->_affDC, &pfd);
    if(_pixelFormat == 0)
    {
        printf("failed to  choose pixel format");
        return false;
    }
     DescribePixelFormat(rc->_affDC, _pixelFormat, sizeof(pfd), &pfd);
    if(SetPixelFormat(rc->_affDC, _pixelFormat, &pfd) == FALSE)
    {
        printf("failed to set pixel format");
        return false;
    }
    rc->_affRC = wglCreateContext(rc->_affDC);

    if(rc->_affRC == 0)
    {
        printf("failed to create gl render context");
        return false;
    }

    return rc;
}
//Call at the end to make it current :

 bool glMultiContext::makeCurrent(renderingContext *rc)
{
    if(!wglMakeCurrent(rc->_affDC, rc->_affRC))
    {
        printf("failed to make context current");
        return false;
    }
    return true;
}
    ////  init OpenGL objects and rendering here :
     ..........
     ............

正如我所说的，我在设备和上下文创建的任何阶段都没有错误。我做错了什么?

更新:

好吧，看来我找到bug了。在调用wglmakecurcurrent()之后，我调用glfwTerminate()，因此看起来最新的"uncurrent"也是新的上下文。虽然它是连接的，因为OpenGL命令不断被执行。所以它在一个线程中工作。

但是现在，如果我使用boost线程生成另一个线程，我就会得到初始错误。下面是我的线程类:

GPUThread::GPUThread(void)
{
    _thread =NULL;
    _mustStop=false;
    _frame=0;

    _rc =glMultiContext::getInstance().createRenderingContext(GPU1);
    assert(_rc);
    glfwTerminate(); //terminate the initial window and context
    if(!glMultiContext::getInstance().makeCurrent(_rc)){
        printf("failed to make current!!!");
    }
             // init engine here (GLEW was already initiated)
    engine = new Engine(800,600,1);
}
void GPUThread::Start(){

    printf("threaded view setup ok");
    ///init thread here :
    _thread=new boost::thread(boost::ref(*this));
    _thread->join();
}
void GPUThread::Stop(){
    // Signal the thread to stop (thread-safe)
    _mustStopMutex.lock();
    _mustStop=true;
    _mustStopMutex.unlock();
    // Wait for the thread to finish.
    if (_thread!=NULL) _thread->join();
}
// Thread function
void GPUThread::operator () ()
{
    bool mustStop;
    do
    {
        // Display the next animation frame
        DisplayNextFrame();
        _mustStopMutex.lock();
        mustStop=_mustStop;
        _mustStopMutex.unlock();
    }   while (mustStop==false);
}

void GPUThread::DisplayNextFrame()
{
    engine->Render(); //renders frame
    if(_frame == 101){
        _mustStop=true;
    }
}
GPUThread::~GPUThread(void)
{
    delete _view;
    if(_rc != 0)
    {
        glMultiContext::getInstance().deleteRenderingContext(_rc);
        _rc = 0;
    }
    if(_thread!=NULL)delete _thread;
}