The "Geopoly" extension mentioned in the link of the original post
supports non-rectangular regions. Each entry in Geopoly is a polygon.
The extension supports primitives to:
* Determine if polygons intersect
* Determine if one polygon is entirely contained inside another
* Determine if a point is inside or outside a polygon
* Find the area of a polygon
* Perform linear transformations (flipping, stretching, rotating)
on polygons.
* Render polygons as SVG.
The inputs are GeoJSON descriptions of polygons. You can search online to
find lots of maps in the GeoJSON format, which can be directly imported
into a Geopoly index.
An SQL script that demonstrates Geopoly's capabilities is in the source
tree in the [ext/rtree/visual01.txt][1] file. Run this file as follows:
~~~~
sqlite3 <visual01.txt >geopoly-demo.html
open geopoly-demo.html
~~~~
For your viewing convenience, I have uploaded the output to
<https://sqlite.org/tmp/geopoly-demo.html>.
[1]: https://sqlite.org/src/file/ext/rtree/visual01.txt