title: Qt opacity color brush?
body: <p>Particular in my case, I want to paint a line to a <em>QGraphicsScene</em> using adLine(). Befor that I have paint something on the scene and I want the last big line on the top of all of these but i still can see what I already paint.  </p>
  
  <p>In general, can I paint an Item and fill it with that kind of color which I can see what under the Item?  </p>
  
  <p>I have read some thing about BGMode here: <a href="http://doc.qt.io/qt-5/qt.html#BGMode-enum" rel="nofollow">http://doc.qt.io/qt-5/qt.html#BGMode-enum</a> but I not sure how it work.  </p>