QGraphicsItems which clip to a shape print with a low resolution edge

Some QGraphicsItems in our scene have the QGraphicsItem::ItemClipsToShape flag set, and we overload QGraphicsItem::shape() to provide this shape information.
We are finding that this works well when displaying the scene on the screen, but when printed the edge of the clipping area appears low resolution and irregular.

We have reproduced this problem in a small standalone example (which I'll attach), on Windows and OS X.
This example generates a very simple scene containing two items. One is a simple hexagonal polygon, and the other is a filled rectangle with a hexagonal clip applied to it. When viewed on the screen both items look identical, but when sent to the printer the clipped item looks more like an irregular 'blob'. See printed.png for an example of what we're seeing.