Fix inside-out polygon artifact in ObliqueMercator projection#2651
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jackbenn wants to merge 1 commit into
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Fix inside-out polygon artifact in ObliqueMercator projection#2651jackbenn wants to merge 1 commit into
jackbenn wants to merge 1 commit into
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When a polygon crosses the projection cut line (the antipodal great circle), _attach_lines_to_boundary can produce an exterior ring that traces the *complement* of the intended shape while still carrying the correct CCW/CW winding flag. The ring is therefore classified as exterior by _rings_to_multi_polygon, and sgeom.Polygon(exterior_ring) produces a polygon covering ~80-95% of the projection domain — causing ocean areas to render as land colour. The fix: before processing each exterior ring in the normal holes-slurping loop, check whether the ring's polygon area exceeds half the projection domain area. If it does, the ring is inverted; replace it with boundary_poly.difference(ring_polygon) to recover the correct shape and continue to the next ring. Regression test added in TestMisc.test_oblique_mercator_no_inverted_land using 50m Natural Earth land data (the bug only manifests at 50m or finer; AdaptiveScaler picks 50m when the map extent is small enough to require more detail). Fixes rendering of land/ocean features in ObliqueMercator projections over the North Pacific (e.g. a Seattle–Tokyo strip map), where several large land masses (Alaska, Chukotka, Kamchatka) cross the cut line. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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QuLogic
reviewed
Jul 15, 2026
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| # ring rather than an interior one). The tell-tale sign is an | ||
| # area that exceeds half the projection domain: a genuine land or | ||
| # ocean polygon can never be that large. Invert such rings back |
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As noted in #2650, I don't know why this assertion holds. Surely if you zoom in enough, any land or ocean polygon can fill up to the entire domain.
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Rationale
When a polygon crosses the projection cut line (the antipodal great circle), _attach_lines_to_boundary can produce an exterior ring that traces the complement of the intended shape while still carrying the correct CCW/CW winding flag. The ring is therefore classified as exterior by
_rings_to_multi_polygon, andsgeom.Polygon(exterior_ring)produces a polygon covering ~80-95% of the projection domain — causing ocean areas to render as land colour.The fix: before processing each exterior ring in the normal holes-slurping loop, check whether the ring's polygon area exceeds half the projection domain area. If it does, the ring is inverted; replace it with
boundary_poly.difference(ring_polygon)to recover the correct shape and continue to the next ring.Regression test added in
TestMisc.test_oblique_mercator_no_inverted_landusing 50m Natural Earth land data (the bug only manifests at 50m or finer;AdaptiveScalerpicks 50m when the map extent is small enough to require more detail).Fixes rendering of land/ocean features in ObliqueMercator projections over the North Pacific (e.g. a Seattle–Tokyo strip map), where several large land masses (Alaska, Chukotka, Kamchatka) cross the cut line.
This is a fix for #2650
Implications
This should have no effect beyond fixing the immediate bug.