Path - Polygon with Holes

To draw a polygon with holes, it is actually not all that much different from drawing regular polyons. The only thing needs to be dealt with is the curve orientation.

If the outer polygon's path is specified in clockwise (CW) ordering, then the inner hole paths must be specified in counter-clockwise (CCW) ordering. Likewise, if the outer polygon's path is CCW, then inner hole paths need to be CW. If not, then the holes will be filled.

The algorithm that determines the curve orientation is pretty simple. See https://en.wikipedia.org/wiki/Curve_orientation. agg::path_storage also has the functionality.

An alternative approach to the path ordering is to use the even-odd filling. See Path - Fill Even-Odd.

SVG's path element has the similar behavior.

Example

tutorial_path_3.cpp generates an image containing red square with a hole.

An equivalent SVG is