QGLWidget tearing when drawing video

[koan]

I'm trying to render video on a GL quad using a QGLWidget but I am experiencing tearing in the video. I'm not an OpenGL expert.

I set the option QGL:oubleBuffers in the constructor and the first thing I do is makeCurrent().

I try to synchronize the application frame rate to the monitor refresh rate, following the advice in http://lists.trolltech.com/qt-intere.../msg00313.html - it's Mac OS X specific unfortunately; I'll repeat it here for completeness:

AGLContext aglContext;
aglContext = aglGetCurrentContext();
GLint swapInt = 1;
aglSetInteger(aglContext, AGL_SWAP_INTERVAL, &swapInt);

AGLContext aglContext;
aglContext = aglGetCurrentContext();
GLint swapInt = 1;
aglSetInteger(aglContext, AGL_SWAP_INTERVAL, &swapInt);

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This helps a little but the tearing is still present.

In the paintEvent I bind the next frame texture. I use a timerEvent to call update and then preload the next frame.

Any help gratefully appreciated.