h3ToGeoBoundary sometimes returns octagon

[usuallycwdillon]

While trying to track down some unexpected behavior from making shapefiles from polyfill objects, I came across an octagon at address '83a350fffffffff'. The error came up while making shapefiles in h3-java, but after finding the same result in h3-python, I compiled today's version of H3 core, and found it there, too.

[isaacbrodsky]

Hi,

I believe this is correct behavior. h3ToGeoBoundary introduces additional vertices if vertices of a cell lie on different icosahedron faces due to projection distortion, which is the case here. This happens in Class III resolutions, which are the odd numbered resolutions. In the extreme case, an H3 cell can have up to 10 vertices.

The cell still has 6 neighboring cells (or 5 for the pentagons at the icosahedron vertices) but has a more complicated edge.

[usuallycwdillon]

Ah. I had read that but hadn't groked it. Thank you.