irr::core::line2df rotate;

bool OnEvent(SEvent &event)
{
    if(event.EventType == MOUSE_INPUT_EVENT)
    {
        if(event.MouseInput.Event == MOUSE_MOVED && rotation)
        {
            rotate.end = device->getCursorControl()->getPosition();
            float rotation = 0; // la tu récupère la rotation 'current'
               rotation += rotate.getLength();
            return true;
        }
        if(event.MouseInput.Event == MOUSE_PRESSED)
        {
            rotate.start = device->getCursorControl()->getPosition();
            rotation = true;
            return true;
        }
        if(event.MouseInput.Event == MOUSE_RELEASED)
        {
            rotation = false;
            return true;
        }
    }
    return false;
}