Point-in-poly for QPainterPath with holes - c++

Question

I'm trying to use a QGraphicsPathItem to represent a polygon with a hole in it (think doughnut). I can get it to draw correctly, and the center is transparent. However, with item selection or movement turned on, I am able to interact with my object when clicking in the hole. I'd rather treat the hole as not part of the polygon.
I've done some testing, and it appears that QPainterPath::contains() will return true when I check a point within the hole. Will I need to subclass QGraphicsPathItem to implement a more specific contains() function, or is there something else I'm missing?

Answer 1

If you change the filling rule from default value Qt::OddEvenFill to Qt::WindingFill, can you still see the hole? I guess you'll not be able to see the hole. So the hole is actually not a "physical" hole in your path. If you are going to represent a polygon with a hole, you may need to subclass the QGraphicsPathItem, explicitly define outer most path and holes, and maintain the relationship among paths in different role.

Answer 2

You'll need to subclass QGraphicsPathItem and reimplement the shape method. Here's what I did to solve the similar problem with an unfilled versus filled rectangle. This particular case is subclassed from QGraphicsRectItem. This also has some adjustment to the path width to make it easier for the user to click on, essentially a buffer zone around the path.
In the case of a filled rectangle, the returned shape is simply a rectangle, but for an unfilled rectangle, it's a stroked path with an empty interior. Then the object only moves when the user drags the edge, but not the middle. Figuring out the QPainterPathStroker wasn't intuitive from the documentation, but it was actually pretty simple to use.
QPainterPath MyRectItem::shape (void) const
{
if (this->brush().style() != Qt::NoBrush)
{
return QGraphicsRectItem::shape();
}
// The rectangle is unfilled. Create a path outlining the rectangle.
QPainterPath path;
QRectF rect = this->rect();
path.moveTo (rect.topLeft());
path.lineTo (rect.topRight());
path.lineTo (rect.bottomRight());
path.lineTo (rect.bottomLeft());
path.lineTo (rect.topLeft());
QPainterPathStroker stroker;
if (this->pen().style() != Qt::NoPen)
{
// For easier selection, increase the pen width to provide a buffer zone
// around the line where the user can click and still select.
stroker.setWidth (this->pen().width() + 20);
stroker.setCapStyle (this->pen().capStyle());
stroker.setJoinStyle (this->pen().joinStyle());
}
return stroker.createStroke (path);
}