Guard degenerate-axis resolution in viewshed CPU sweep#2745
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_viewshed_cpu computed ew_res/ns_res as range/(dim-1), which is 0/0 = NaN for a single-row or single-column raster. The NaN propagated through all distance/gradient math, silently marking every non-observer cell INVISIBLE on flat terrain. Fall back to unit resolution for a degenerate axis, matching _viewshed_windowed and _viewshed_dask. The max_distance window path routes a degenerate window back through _viewshed_cpu, so it is fixed too. Adds regression tests for 1xN and Nx1 rasters on both the full and max_distance paths.
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May 30, 2026
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PR Review: Guard degenerate-axis resolution in viewshed CPU sweep
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What looks good
- The fix matches the guard already used in
_viewshed_windowed(viewshed.py:2078-2079) and_viewshed_dask(viewshed.py:2180-2181), so the three CPU-resolution computations now agree. - Falling back to
1.0for a length-1 axis is the right call: a single row has no defined y-resolution, and unit resolution keeps the distance/gradient geometry well-defined. - Tests cover both orientations (1xN and Nx1) and both code paths (full sweep and
max_distancewindowing, which delegates back to_viewshed_cpu). The assertions check the real failure mode: observer cell at 180, all other cells visible, and no NaN in the output. - I checked the 1x1 case by hand (both axes degenerate): it returns
[180.]without error, so the two guards compose correctly.
Checklist
- Algorithm matches reference/convention (mirrors sibling helpers)
- NaN handling is correct (the fix removes the NaN source)
- Edge cases covered by tests (1xN, Nx1; 1x1 verified manually)
- No premature materialization or unnecessary copies
- Benchmark not needed (bug fix, no perf change)
- README feature matrix not applicable (no API change)
- Docstrings unchanged and still accurate
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Closes #2744
_viewshed_cpucomputed grid resolution asrange / (dim - 1). For a single-row or single-column raster that is0 / 0 = NaN, which then flowed through every distance and gradient calculation and silently marked every non-observer cell INVISIBLE on flat terrain (no error raised)._viewshed_windowedand_viewshed_dask.max_distancepath routes a degenerate window back through_viewshed_cpu, so it is covered by the same fix.Backend coverage: numpy (direct fix). The dask and cupy/RTX entry points already guarded this, so their behavior is unchanged; the windowed path that delegates to the CPU helper is now correct on degenerate inputs.
Test plan:
test_viewshed_single_row_or_column(1x5 and 5x1) — observer cell is 180, all cells visible, no NaNtest_viewshed_single_row_or_column_max_distance(1x7 and 7x1) — windowed path correcttest_viewshed.pysuite passes (29 passed)