d3.geoCentroid sometimes returns the wrong result?

[mbostock]

https://bl.ocks.org/mbostock/c62c0d706d1f046f84c0f0cf698bc0a9

Moved from d3/d3#1883. /cc @Fil

[mbostock]

Support for area-weighted polygon centroids was added in D3 3.2. Related d3/d3@492215a d3/d3#1011 d3/d3#1286. Interestingly, the prior behavior is more accurate for this input.

[Fil]

in function centroidRingPoint(lambda, phi) { if I change v = m && -acos(u) / m, // area weight to v = -1 the blue dot ends up on the red dot.

[Fil]

if I apply this patch then some tests fail, but by not much — moreover, the failing tests are weird:

# the centroid of a spherical square touching the antimeridian
not ok 124 should be in delta 0.000001
  ---
    operator: inDelta
    expected: |-
      [ -179.5, 4.987448 ]
    actual: |-
      [ -179.50000000000028, 4.999810580369795 ]
    at: Test.tape.Test.inDelta (/Users/fil/Sites/d3/d3-geo/test/inDelta.js:5:8)
  ...

the actual result (with v=-1) looks much closer to what I understand to be the centroid of said "circle" (which doesn't look circular at all).

(Looks like we're not testing features but non-regression.)

[mbostock]

I dunno, sorry.

[Fil]

Fix is live at https://bl.ocks.org/Fil/8090c35ded3e46368843dfed82543617

[mbostock]

Released in 1.4.2.

[Fil]

I took the time to write this up so that the next person who wants to understand the code might spend less time on it. (In any case it wasn't time wasted.)
https://bl.ocks.org/Fil/608b3ca94a008f2ae01c6273fafb25e7