title: OpenGL background transparency?
body: <p>I'm using QT's QGLFramebufferObject for off-screen rendering.<br>
  After rendering to the buffer I read the result using glReadPixels()<br>
  The problem is that sometimes the background color I read is just 0 (transparent black) and sometimes it is 0xFF000000 (opaque black)<br>
  This seem to be related to the time the buffer is initialized. If the buffer is a member of the class inherited from QGLWidget then it is 0. If it is initialized anywhere else it's 0xFF000000.  </p>
  
  <p>Does anybody have any idea what's going on here? What is the good expected result of this read?</p>