Densify the geostationary bounding polygon before reprojecting in AreaSlicer

[lorenzo-cingano]

AreaSlicer.get_polygon_to_contain builds the bounding polygon of a geostationary source disk and reprojects it into the destination projection to compute the source slice. When the source is a partial disk (rapid scan or region of interest data), clipping the disk to its area extent introduces straight chord edges with only two vertices. Those chords are not straight in the destination projection, so the reprojected polygon cuts the corner and the source slice comes out too small, leaving part of the destination empty. A full disk source is unaffected, because its whole boundary is the densely sampled disk arc.

This densifies the polygon with segmentize before reprojecting it, so the chord edges keep enough vertices to follow their true shape in the destination projection. The problem was found in satpy when resampling MSG rapid scan data to a polar stereographic area with gradient_search.

segmentize needs shapely 2.0 or newer. shapely is currently unpinned in the dependencies, so a shapely>=2.0 pin can be added if you want it explicit.

• Closes gradient_search leaves part of the destination empty for a partial geostationary disk #728
• Tests added
• Tests passed
• Fully documented

[djhoese]

Thanks for this PR! I just wanted to let you know that we're not ignoring you, but most of us are on vacation or (like me) don't have time to review this right now. I hope to review it next week though.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (2e7a3c8) to head (1065291).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #729      +/-   ##
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+ Coverage   93.68%   93.90%   +0.22%     
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  Files          89       89              
  Lines       13715    13937     +222     
==========================================
+ Hits        12849    13088     +239     
+ Misses        866      849      -17     

Flag	Coverage Δ
unittests	93.90% <100.00%> (+0.22%)	⬆️

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[Manny7717]

Review

Verified locally on head 99f9c93c: 19/19 tests in pyresample/test/test_slicer.py pass (shapely 2.1.2). The approach is sound — densifying in the source projection before the inverse transform is exactly where the straight chord edges need intermediate vertices, since chords in the geos projection become curves in the destination CRS.

Two things to look at (inline): a Shapely minimum-version concern for segmentize, and a degenerate-extent crash in the segment_length computation. Both are cheap to guard.

[djhoese]

Thanks for the work on this @lorenzo-cingano. I'm going to merge this now but then borrow your solution in my new #757 PR where it is applied to all area slicing.