What is the fastest way to draw a circle ?

[wysota]

So I'll probably stick with using GL. But what if user has not OpenGL (I've tried to run my program in Xephyr and got just error messages). Are there any ways to check it at runtime ?

QGLFormat::hasOpenGL()
QGLFormat::sampleBuffers()

Thanks for the advice ! But when using QPixmap I'm getting very noticeable jitter when particle moves because QPixmap can only be drawn at integer position. Are this problem fixed with the patch you mentioned ?

You can speed up pixmap scaling by tweaking transformation modes from smooth to fast. You can improve quality by doing the opposite thing. You'll always have aliasing when using pixmaps, because they are pixel based (using smooth transform mode will reduce the effect). But if you use GL pixel buffers and render the circle to texture, you should avoid any artifacts at all. Should be very simple in your case.