_tile_perimeter returns 17 points: why ?

[elpaso]

Hi,

I'm still working on simplify-aware Vector driver (see my branch on
github, see also my previous post on this list), I noticed that after
clipping I still had many points in my geometries, this is due to the
fact that the clipping bbox (which is a rectangle) does not have only
4 (+1 if you need the closing point) coordinates like rectangles
normally have but 16 (+1) !!!

as written in the docs
_tile_perimeter(coord, projection, padded)
returns 17 points

can somebody please explain why and if those extra points are really
needed ?

Most of my efforts to reduce the size of the generated json are
vanished by those apparently unuseful points.