South aspect of Lee’s conformal projection in a tetrahedron is all black

[espinielli]

South aspect of Lee’s conformal projection in a tetrahedron shows all black.

From the tweet exchange @Fil has verified that

it doesn't happen with d3-geo-polygon@1.7 but with d3-geo-polygon@1.8

[Fil]

Ref: https://observablehq.com/@fil/lee-projection (which I momentarily “fixed” by going back to d3-geo-polygon@1.7).

[Fil]

the bad commit is eb1e663 (of course)

[Fil]

There is this quick test in https://github.com/d3/d3-geo-polygon/blob/master/src/intersect.js#L19 : two arcs of angles a.l and b.l can not intersect if the distance between start (or end) of the first and the start (or end) of the second make an angle > a.l + b.l.

I optimized it with the acos of the angles and forgot that, if A+B > pi, the cosine increases again. Fix and test incoming.

[espinielli]

you are a wizard!
well done and thanks