How can I use glReadPixels without hWnd? - windows

Question

I succeeded to read pixels from DC which OpenGL rendered on.
Here is a part of my code.
void COpenGLWnd::OffscreenRender
(/* IN parameters */ int transitionID, int counts, int directionID,
/* OUT parameters */ BYTE* pPixelArray)
{
HDC hDC = ::GetDC(m_hWnd);
SetDCPixelFormat(hDC);
HGLRC hRC = wglCreateContext(hDC);
VERIFY(wglMakeCurrent(hDC, hRC));
for(int i = 0; i < counts; i++)
{
GLFadeinRender((GLfloat)(i+1) / (GLfloat)counts);
BYTE* data = (BYTE*) malloc(m_BmpSize);
glReadBuffer(GL_BACK);
glReadPixels(0,0,m_BmpWidth,m_BmpHeight,GL_BGR_EXT,GL_UNSIGNED_BYTE, data);
memcpy(&pPixelArray[i * m_BmpSize], data, m_BmpSize);
free(data);
}
wglMakeCurrent(hDC, NULL);
wglDeleteContext(hRC);
::ReleaseDC(m_hWnd, hDC);
}
It works very well, but I want to use glReadPixels without hWnd. I heard that PBO or FBO may help for this situation but I cannot make it. Here is the code that I tried to render with PBO.
void COpenGLWnd::OffscreenRender
(/* IN parameters */ int transitionID, int counts, int directionID,
/* OUT parameters */ BYTE* pPixelArray)
{
GLuint pbo;
glGenBuffersARB(1,&pbo);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo);
glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB, m_BmpSize, NULL, GL_STREAM_READ_ARB);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);
for(int i = 0; i < counts; i++)
{
glReadBuffer(GL_BACK);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo);
GLFadeinRender((GLfloat)(i+1) / (GLfloat)counts);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pbo);
//glReadBuffer(GL_BACK);
glReadPixels(0,0,m_BmpWidth,m_BmpHeight,GL_BGR_EXT,GL_UNSIGNED_BYTE, 0);
BYTE* data = (BYTE*) glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
memcpy(&pPixelArray[i * m_BmpSize], data, m_BmpSize);
if(data)
{
glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB);
}
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB,0);
}
glDeleteBuffers(1,&pbo);
}
But nothing happened at glReadPixels. No pixels read.
So can I off-screen render and read pixels without using hWnd?