region createCircularRgn(int x, int y, int d)
Returns a handle/pointer to a circular region object (see comments)
Allows to define a rounded circular visual region, that can be for instance handled by the functions refreshRgn or setDialogRegion.
setDialogRegion( createCircularRgn(20, 20, 200) );
"region" corresponds to a particular FM type or object that might need to be better documented or defined. You can't directly cast it to an integer, despite it behaving like an integer.
createCircularRgn is a wrapper around the Windows Win32 built-in function CreateEllipticRgn, documented here: https://msdn.microsoft.com/en-us/library/windows/desktop/dd183496(v=vs.85).aspx
createEllipticRgn, createRectRgn, createRoundRectRgn, createCircularRgn, createPolyRgn, refreshRgn, setDialogRegion