May I ask you another question. I planned to use a QLabel for displaying pixmaps (each 40 ms). From your experience, is it slower than your method, ie using QImage and QBrush-> fillRect? Either, did you try the drawPixmap function of Qpainter?
I didn't try that under Qt4 so I can't tell you how fast it is. (my method).
Or if it even works.
To what you had in mind:
In case you are getting a buffer from external source - like a camera, you will have to convert from QImager to QPixmap - and that is slow.
Your aim should be to feed your buffer as directly as you can to the paintEvent().