Solved.

For further readers:
I've added a check to the child widget whether a user interaction happens within its drawn borders (a closed QPainterPath).
If so, I consume the event and continue from there, if not, I ignore the event and pass it to the parent widget, i.e. the actual MyWidget, so that the overlaying child widget has some transparency for my defined events (some key and mouse interaction).

Pretty easy if I think about it now.
The idea of having some kind of a donut shaped widget (yellow part in my sketch), which does not occupy the center of its parent widget was stuck in my head way too long :))

Try QPainterPath::simplified()

thank you for the direction.

@Pl45m4 True, I see that QPainterPath is getting me nowhere in this case. Thanks.

@Niagarer
Good found. :)

@Joel-Bodenmann Two points: 1. You can decompose the path to get each element and work on those independently. 2. You must contend with lines, polylines, ellipses and Bezier splines.

Closes point on a line is simply a vector projection. Bezier splines are more complex. For an approximation you can get polyline approximation to the remainder using parametric equations, then use the same technique for lines. I am writing a book on this subject.

By the way, I recall seeing an article on the method you propose.

There is a reason why there are almost no usable free implementations out there: Because it's harder than one would think at first sight. Maybe start which Shewchuk's papers to get a first idea of the problem.

@Wieland ,nice .......it works ,thanks for reply:)

@Wieland it is example of Qt ,its name diagramscene thanks for reply

@mrjj Well, thanks to you and @kshegunov too. You guys are amazing!

Hi and welcome
It has an overloaded + operator so you can just use that.
Note: This does not delete it as such, it combines.

void SWidget::paintEvent ( QPaintEvent* e ) { QPainter p ( this ); QPainterPath path1; path1.addRect ( 20, 20, 60, 60 ); QPainterPath path2; path2.addEllipse ( 40, 40, 60, 60 ); QPainterPath path3 = path1 + path2; p.drawPath ( path3 ); }