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fix(analysis): keep polygon simplification monotonic at large tolerances - #911
Simplify could return more vertices at a higher tolerance because the polygon RDP implementation restored original segments when removing them would leave fewer than four ring coordinates. A regression reproduced 85 → 110 vertices when increasing the tolerance from 100 m to 500 m.
Anchor three original, non-collinear vertices in each polygon ring and apply the existing line Douglas–Peucker implementation to the arcs between them. The anchors form the minimum closed triangle at large tolerances. Increasing the tolerance no longer restores the full ring. Preserve ring order, holes and multipart ring counts, and reuse simplify_geometry in the geoprocessing tool so it shares the fix with layout generalization and mobile packages.
The publication locator retains its existing policy of keeping sub-tolerance island rings unchanged; the tool itself continues to simplify them. Ring-size measurement uses continuous longitudes across the antimeridian, so small dateline-crossing islands retain their original coordinates too.
No dependency or parameter changes. The default remains 25 km. The user guide explains the minimum ring and the existing topology limitation: this is not a topology-preserving simplifier, so holes and shared boundaries still need checking.
Testing
Regression failed before the fix and passed after it.
Headless publication export with a globe locator rendered and visually inspected.
Regression coverage includes increasing tolerances, distance bounds, closed rings, holes/multipart geometry, degenerate inputs, attributes, source preservation and undo/redo. A new locator regression covers ordinary islands, antimeridian crossings and high latitudes. The existing publication locator regression also passed unchanged.
Polygon simplification better preserves valid rings, holes, and small boundaries at higher tolerances.
Simplifying geometry retains small polygons in the globe locator, including near the date line.
Documentation
Clarified how Simplify handles polygon rings and holes, its kilometre-based tolerance, and its lack of topology preservation. Added guidance on choosing tolerances for local boundaries.
No actionable comments were generated in the recent review.
Recent review info
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Reviewing files that changed from the base of the PR and between c0c0cbf and d929284.
Files selected for processing: 2
crates/engine/src/cmd/layout.rs
crates/engine/src/tests.rs
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Walkthrough
Polygon simplification now processes exterior and interior rings separately. The globe locator restores individual small rings. Tests cover ring validity, vertex counts, output stability, undo and redo, and small-island geometry. The user guide describes the algorithm and its tolerance.
Changes: Polygon Simplification
Layer / File(s)
Summary
Ring simplification crates/analysis/src/tools.rs, docs/user-guide/analysis.md
Polygon simplification now simplifies each ring through anchored arcs and delegates geometry handling to simplify_geometry. Tests check ring validity, preserved parts, deviation limits, and vertex counts across tolerances. The guide describes the algorithm, tolerance, and polygon-ring behavior.
Locator and integration crates/engine/src/cmd/layout.rs, crates/engine/src/tests.rs
The locator restores individual rings when their latitude-adjusted projected extent is below tolerance. Tests check simplify output attributes, source preservation, undo and redo, and preservation of small locator polygons.
opengeos/RustGIS#706: Adds the publication globe locator whose polygon sketch uses the simplification behavior changed here.
Suggested reviewers:giswqs, elpidioassinaturas
Merge Risk: Minimal · up to d9292
The reviewed change is mergeable after normal checks; no unresolved issue from this review requires a fix.
Pre-merge checks: 4 · 1
Failed checks: 1 inconclusive
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
Status
Explanation
Title check
The title clearly and concisely identifies the main change: maintaining monotonic polygon simplification at larger tolerances.
Description check
The description includes complete Summary, Testing, and Related issue sections. It explains the problem, implementation, validation, and linked issue.
Linked Issues check
Issue #895 requires that vertex counts do not grow when tolerance increases. The PR anchors three original non-collinear vertices in each polygon ring and applies Douglas–Peucker to the ring arcs. Lar…
Out of Scope Changes check
The changes remain within Issue #895. The analysis change implements polygon simplification. The analysis tests verify vertex-count stability and related behavior. The publication locator change prese…
Full details: Linked Issues check
Explanation
Issue #895 requires that vertex counts do not grow when tolerance increases. The PR anchors three original non-collinear vertices in each polygon ring and applies Douglas–Peucker to the ring arcs. Large tolerances therefore produce the minimum valid closed triangle instead of restoring the original ring. The simplify tool delegates to simplify_geometry, so the geoprocessing path uses the fix. Regression coverage checks increasing tolerances, rings with holes and multipart geometry, ring validity, and tolerance bounds. The optional alternative of reporting unchanged features is not required because the issue accepts the smallest valid ring.
Full details: Out of Scope Changes check
Explanation
The changes remain within Issue #895. The analysis change implements polygon simplification. The analysis tests verify vertex-count stability and related behavior. The publication locator change preserves its existing policy for sub-tolerance island rings after the simplifier changes. The locator regression covers ordinary, antimeridian, and high-latitude islands. The guide documents the new minimum-ring behavior and the existing topology limitation. No unrelated dependency, parameter, or public API change is reported.
Full details: Docstring Coverage
Explanation
Docstring coverage is 22.22% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 2 files. (1 skipped: 1 too large.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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A rabbit checks each ring with care The corners stay, the holes stay there A larger tolerance trims the line Small island rings remain in time The guide records what changes show Then off through fields the rabbits go
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The ring size is computed with an ad-hoc formula. extent_to_merc returns Mercator metres, which are already stretched by 1/cos(lat). Multiplying the diagonal by cos(lat) (floored at 0.01) approximates ground metres, but the comparison is against tolerance_m, which simplify_geometry scales with merc_scale(feature_lat(g)). The two lat conventions can differ (extent centre of one ring vs. the feature's lat), so the cutoff is only approximate. Also, a ring of 0.5–1× tolerance is still reduced to a triangle.
Suggestion: compare the ring in the same units as simplify_geometry (planar diagonal vs. eps), or expose a helper from rustgis_analysis so the "too small to simplify" rule lives in one place. Confidence: low–medium.
The zip over original.iter().flatten() and reduced.iter_mut().flatten() relies on simplify_geometry preserving ring/part structure. That is true today, but a silent mismatch would only truncate the zip. A debug_assert on the lengths, or doing this inside simplify_ring callers, would be more robust.
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simplify_ring pins three anchor vertices (first, farthest, max-area) in every ring. This guarantees at least a triangle, but it changes Douglas–Peucker semantics. Even a tolerance far larger than a ring leaves a triangle, and the anchors are kept even when they are within tolerance of the chord. Holes also collapse to triangles and can now cross the exterior or each other, which the docs acknowledge. With max_by and ties, the anchor choice is deterministic but arbitrary.
Edge case: ring.0.len() <= 4 returns early, so rings with exactly 5 points, such as a closed quadrilateral, are simplified, but unclosed rings (first != last) are not special-cased. The len() - 1 stop assumes closure. Input is probably closed by to_multipolygon, so this is low risk. Confidence: low.
The simplify tool previously reported 0 vertices for geometries that failed conversion and wrote a None geometry. It now clones the original (via simplify_geometry's fallback). That seems better, but it is an observable behaviour change worth noting in the PR.
The change keeps polygon rings from growing back as tolerance rises. I read the diff and the surrounding simplify_geometry code. I did not build or run the tests. I found no crash-class problems, and the changed code follows the "never crash" rules: it uses get() and let ... else, with no unwrap or panic! outside tests.
Bugs
Anchor vertices and unclosed rings (low).simplify_ring pins three anchor vertices per ring, and the last stop is len() - 1, which assumes the ring is closed. The function does not check that, though upstream conversion probably guarantees it. Anchors are pinned even when they lie within tolerance of the chord, so holes always end as triangles and can cross the exterior ring. The docs say the simplifier is not topology-preserving.
Small-ring cutoff in layout.rs (low–medium). The "too small to simplify" cutoff in the locator sketch uses its own formula: the Mercator diagonal scaled by cos(lat) (floored at 0.01). It is compared with tolerance_m, which simplify_geometry converts using merc_scale(feature_lat(g)). These two latitude conventions can differ, so the cutoff is only approximate. Rings between one and two times the tolerance are still reduced to triangles.
Zipped ring lists (low). The zip over original and reduced rings relies on simplify_geometry preserving the part and ring structure. A mismatch would silently truncate the zip.
Security
Nothing found.
Performance
Nothing significant. Anchor selection is O(n) per ring.
Quality
Behaviour change in simplify (info). Conversion failures now clone the original geometry, because the tool delegates to simplify_geometry. Before, they produced a None geometry and counted 0 vertices. This is arguably better, but it is not mentioned in the PR.
Duplicated rule. The small-ring rule now lives in the engine layout code instead of next to simplify_geometry. Exposing it from rustgis_analysis would keep it in one place.
Tests. Coverage is good: monotonic vertex counts, degenerate, collinear, non-finite and concave rings, a hostile-parameter-style check, and engine-level undo/redo.
AGENTS.md
Documentation was updated for the new behaviour.
No new dependencies, no UI text, no wasm concerns.
I could not confirm that cargo xtask ci passes.
Change size
Compared: the whole PR against main (merge base 4a33d80 to PR head c0c0cbf)
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Actionable comments posted: 1
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Inline comments:
Review comments at @crates/engine/src/cmd/layout.rs:
- Around line 1451-1454: Update the ring-size calculation around
`extent_to_merc` to measure antimeridian-crossing rings in continuous longitude
space, unwrapping the ring coordinates before computing the projected extent.
Keep the existing `tolerance_m` comparison, so the restored-outline behavior is
triggered when the continuous-space size is below the threshold.
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Commits
Reviewing files that changed from the base of the PR and between c7bf8e1 and c0c0cbf.
Files selected for processing: 4
crates/analysis/src/tools.rs
crates/engine/src/cmd/layout.rs
crates/engine/src/tests.rs
docs/user-guide/analysis.md
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Low-confidence concern: this zips the flattened rings of original with those of reduced by position, so it relies on simplify_geometry keeping the ring count and order. to_multipolygon/from_multipolygon do a round trip, and I didn't check whether that can drop, reorder or regroup rings. That could be the case for a degenerate ring or a hole that isn't contained in its shell. If the order ever differs, a small ring could be replaced by an unrelated ring's original, or a large one left simplified. A cheap guard is to skip the override unless the two ring counts match, e.g. original.iter().flatten().count() == reduced.iter().flatten().count(). The antimeridian test only covers single-ring islands, so it doesn't exercise this.
The PR replaces the polygon simplifier with a per-ring version that keeps three fixed anchors, so a small ring can no longer collapse or regrow. It also makes the simplify tool call simplify_geometry, and keeps small islands intact on the publication locator. I read the diff only and did not build the code or run the tests.
Bugs
Medium-low confidence:crates/engine/src/cmd/layout.rs:1449 pairs the rings of original and reduced by position. This only works if simplify_geometry keeps the ring count and order. If a degenerate ring or a hole outside its shell is dropped or regrouped, a ring could be swapped for the wrong original. I posted an inline comment with a ring-count guard. The new antimeridian test only uses single-ring islands, so it doesn't cover this.
Security
Nothing found. The new code stays within the no-panic rules: it uses get, let ... else, and a finite-coordinate check, and it has no unwrap or panic! outside tests.
Performance
Nothing significant. simplify_ring clones a ring on its early returns and does three linear scans, which is acceptable.
Quality
Low:simplify now calls simplify_geometry, which removes the duplicated match, and the fallback for other geometry types appears to be unchanged. This is a good cleanup, but I did not read the tail of simplify_geometry to confirm that non-polygon and non-line geometries are cloned rather than dropped.
Info: the ring is split into three arcs at fixed anchors, so a ring is never reduced below a triangle and the result is no longer topology-preserving. docs/user-guide/analysis.md states both limits.
AGENTS.md
The change follows the guidelines I checked: no unwrap or panic! in non-test code, regression tests for the fix, and a docs update.
I didn't see any UI strings or dependency changes, so the tl! and deny.toml rules look not applicable.
Change size
Compared against: the whole PR against main (merge base 4a33d80 to PR head d929284).
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Summary
Simplify could return more vertices at a higher tolerance because the polygon RDP implementation restored original segments when removing them would leave fewer than four ring coordinates. A regression reproduced 85 → 110 vertices when increasing the tolerance from 100 m to 500 m.
Anchor three original, non-collinear vertices in each polygon ring and apply the existing line Douglas–Peucker implementation to the arcs between them. The anchors form the minimum closed triangle at large tolerances. Increasing the tolerance no longer restores the full ring. Preserve ring order, holes and multipart ring counts, and reuse
simplify_geometryin the geoprocessing tool so it shares the fix with layout generalization and mobile packages.The publication locator retains its existing policy of keeping sub-tolerance island rings unchanged; the tool itself continues to simplify them. Ring-size measurement uses continuous longitudes across the antimeridian, so small dateline-crossing islands retain their original coordinates too.
No dependency or parameter changes. The default remains 25 km. The user guide explains the minimum ring and the existing topology limitation: this is not a topology-preserving simplifier, so holes and shared boundaries still need checking.
Testing
cargo xtask ci: passed (2,245 tests passed, 16 ignored; formatting, Clippy and layer checks passed).uvx pre-commit run --all-files: passed.cargo build --locked --target wasm32-unknown-unknown -p rustgis-web: passed.Related issue
Closes #895.
Summary by CodeRabbit