suppose I use the QGLWidget's paintGL() method to draw into the widget using OpenGL. After the Qt called the paintGL() method, it automatically triggers a buffer swap. In OpenGL, this buffer swap usually blocks the calling thread until the frame rendering to the background buffer is completed, right? I wonder which Qt thread calls the paintGL as well as the buffer swap. Is it the main Qt UI thread? If it is, wouldn't that mean that the block during the buffer swap also blocks the whole UI? I could not find any information about this process in general..
Thanks
I don't use the QGLWidget very often, but consider that yes, if swapBuffers() is synchronous the Qt GUI thread is stuck. This means that during that operation you'll be unable to process events.
Anyway, if you're experiencing difficulties while doing this, consider reading this article which manage to allow multithreaded OpenGL to overcome this difficulty.
Even better, this article explains well the situation and introduces the new multithreading OpenGL capabilities in Qt 4.8, which is now in release candidate.