QGraphicsItem/QPainter opacity on mac is inconsistent?

[davidb]

I'm using Qt 4.8.4. I have subclassed QGraphicsItem and in the paint method:
@painter->setOpacity(0.30);@

then I create a QRectF contrast and
@painter->drawRect(contrast);@

This works fine on Windows 7: The appearance on the screen, printing, and rendering the scene to a QImage to write to a jpg all produce the same opacity of 0.30. On OSX mountain lion 10.8.2, rendering the scene to a QImage to write a jpg works fine as well - it produces identical results to Windows 7.

However, on OSX 10.8.2, this same code produces a much lower opacity on the monitor and printer - probably closer to 0.10 opacity. Obviously, I need what is printed and what shows up on the monitor to match the jpg that is produced. Any idea what's going on? Is this a bug in some Mac-specific part of Qt?