Mollweide projection for polygons that intersect a pole

[xylar]

First let me say that I'm very happy with cartopy. Most projections I have tried handle polygons that that intersect a pole surprisingly well. The one exception I've encountered is the Mollweide projection, which seems to get confused when when the polygon is closer to the pole than 89.0 N. Below is a script that reproduces the issue with a polygon representing the Arctic Ocean basin.

A plot with a PlateCarree projection behaves as expected:

Mollweide produces the following glitch with a polygon that intersects the pole

but behaves as expected if the polygon doesn't quite reach the pole (maxLat = 89 in the script).

import numpy
import matplotlib.pyplot as plt
import cartopy.crs
import cartopy.feature

#maxLat = 89.
maxLat = 90.

points = [
    [ -114.848345, 77.854709],
    [ -120.812209, 76.737200],
    [ -180.000000, 71.535845],
    [ -180.000000, maxLat],
    [ 0.000000, maxLat],
    [ 180.000000, maxLat],
    [ 180.000000, 73.187262],
    [ 173.916445, 73.187262],
    [ 88.445563, 81.255838],
    [ 44.860000, 80.613455],
    [ 24.365000, 80.289982],
    [ 21.798055, 80.146945],
    [ 18.443464, 80.181091],
    [ -28.773336, 83.513609],
    [ -36.783487, 83.573498],
    [ -72.648900, 82.746645],
    [ -76.102782, 82.470536],
    [ -78.843609, 82.664991],
    [ -80.909164, 82.156645],
    [ -83.001955, 82.089155],
    [ -85.405836, 82.042209],
    [ -88.113618, 82.090545],
    [ -93.687500, 81.210264],
    [ -94.093609, 80.593318],
    [ -114.848345, 77.854709]]

points = numpy.asarray(points)


for projection in [cartopy.crs.PlateCarree(), cartopy.crs.Mollweide()]:
    plt.figure()

    ax = plt.axes(projection=projection)
    resolution = '50m'
    ax.add_feature(cartopy.feature.NaturalEarthFeature('physical', 'land', resolution,
                   edgecolor='face', facecolor=cartopy.feature.COLORS['land']), zorder=1)
    ax.add_feature(cartopy.feature.NaturalEarthFeature('physical', 'coastline', resolution,
                   edgecolor='black', facecolor='none'), zorder=2)


    plt.tight_layout()

    ax.fill(points[:,0], points[:,1], transform=cartopy.crs.PlateCarree(), color='green', alpha=0.4, linewidth=2.0, zorder=3)

plt.show()

[QuLogic]

For future reference, the downstream ticket shows that decreasing the threshold had no effect.

[xylar]

@QuLogic, that's correct. It might be easiest if I repeat the snippet I tried here for clarity. At @pelson's suggestion, I made a new child class of Mollweide with a threshold smaller than the default value (1e5). I used a threshold of 1e4 and then 1e3 but the poles still seem to give the Mollweide plot the same difficulty.

class BetterMollweide(cartopy.crs.Mollweide):
    def __init__(self, *args, **kwargs):
        cartopy.crs.Mollweide.__init__(self, *args, **kwargs)
        self._threshold = 1e3

    @property
    def threshold(self):
        return self._threshold