d3-geo-projection
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d3
d3.geoStitch may promote polygon holes to exterior rings.
Consider the case of two polygons in a MultiPolygon, each with their own distinct hole, where the line BC is on the antimeridian:
A-----------------B-----------------J
| | |
| | |
| E-----F | K-----L |
| | | | | | |
| | | | | | |
| H-----G | N-----M |
| | |
| | |
D-----------------C-----------------I
The first polygon starts with these fragments:
A-----------------B
|
|
| E-----F
| | |
| | |
| H-----G
|
|
D-----------------C
The second polygon starts with these fragments:
B-----------------J
|
|
K-----L |
| | |
| | |
N-----M |
|
|
C-----------------I
When ring DBCA is merged with CIJB, I believe that the first polygon retains ring EFGH. However, the other ring, KLMN, is considered a “standalone ring” not belonging to any exterior ring, and thus two polygons result from stitching:
A-----------------B-----------------J
| |
| |
| E-----F |
| | | |
| | | |
| H-----G |
| |
| |
D-----------------C-----------------I
K-----L
| |
| |
N-----M
Furthermore, since KLMN has the opposite winding order, it now covers most of the sphere rather than just be a hole inside BADCIJ.
This problem can be avoided by putting every ring in a MultiPolygon into a single Polygon, as demonstrated here:
https://bl.ocks.org/mbostock/83c0be21dba7602ee14982b020b12f51
However, collapsing a Polygon into a MultiPolygon isn’t valid GeoJSON, since it means that the Polygon may have multiple exterior rings. This only works because d3-geo appears to not be dependent on GeoJSON ring semantics, at least for rendering, relying only on winding order.
Your example at https://beta.observablehq.com/@fil/stitching-multipolygons
It seems that d3.geoProject() has the same issue, where MultiPolygons will "explode" and must be transformed back to polygons. (See https://beta.observablehq.com/@fil/d3-vector-tiles-wip#res).
In geoProject the choice of having a ring be a polygon or a hole is made with https://github.com/d3/d3-geo-projection/blob/master/src/project/clockwise.js, which is definitely planar.
[Addendum] https://github.com/d3/d3-geo-projection/blob/master/src/project/index.js#L114 lists issue 1558 which has disappeared, and explains a lot https://webcache.googleusercontent.com/search?q=cache:5UIKWV7HNFYJ:https://github.com/d3/d3/issues/1558
More later…
