Hi Divya,

The easiest way to test which region is an aperture and which is an obscuration is to place the UDA on a flat surface and trace a grid of rays. You can't really tell just from the graphic. First of all it depends if the UDA is loaded as an aperture or an obscuration: that is separate to the definition of the UDA itself. So you can 'reverse the polarity of the UDA' with that switch.

Multiple polygons may also be nested inside a UDA. If a ray intercepts a point within a polygon that is within another polygon, then the point is considered to be outside the aperture. This convention allows “islands” to be defined within apertures that become obscurations, and vice-a-versa. Any number of nesting levels is allowed, and each level toggles the inside/outside status of the point.