[Qt-interest] Update region granularity

[Bob Hood]

I have a QDialog with various child widgets.  I am wanting to capture paint
updates to them, so I install an eventFilter for each widget in the QDialog
constructor, thus:

    QList<QWidget *> widgets = findChildren<QWidget *>();
    foreach(QWidget* widget, widgets)
        widget->installEventFilter(this);

In the event filter, I trap paint events:

    if(event->type() == QEvent::Paint)
    {
        QPaintEvent* paint_event = (QPaintEvent *)event;
        if(paint_event)
        ...

And then I grab and cache the update by iterating over all the rectangles that
make up the region:

            ...
            QWidget* widget = (QWidget*)object;
            foreach(const QRect rect, paint_event->region().rects())
            {
                QPoint p = widget->mapTo(this, rect.topLeft());
                QRect r = QRect(p, rect.size());
                QString key = QString("%1x%2x%3x%4")
                               .arg(r.x())
                               .arg(r.y())
                               .arg(r.width())
                               .arg(r.height());
                capture_list[key] = r;
            }
            ...

This approach always seems to end up giving me a granularity that is no less
than the entire face of the widget.  If it's a slider, for example, I end up
with rectangles derived from the QPaintEvent region() that represent the
entire slider, not just the portion of the slider that changed -- i.e., the
handle and the areas to the left and right of it.

My question:  Is this the best granularity I can hope for by trapping updates
at the QDialog level (the "whole widget" granularity)?  Can I only get
specific damage regions on individual widgets by sub-classing all of them and
re-implementing their (re)paint methods?  This latter solution is really not
practical, because I really don't even know what the population of the QDialog
consists of, let alone know all their types and have to code sub-classes for
them all.

I can't go into a lot of detail about why I am doing this, but the goal is to
send the smallest amount of data over a network to refresh a GUI elsewhere. 
If I'm stuck with the "whole widget" as the smallest amount, I suppose that
okay.  I just want to make sure there's not something else I'm overlooking.

Thanks!