[pmeems]

Motivation

The OpenGL backend only accepts convex single-ring polygons and everything else falls back to GDI. This PR is phase 1 of the polygon triangulation plan: provide reusable, backend-neutral triangulation in Core so phase 2 can render concave polygons, holes, and multipart geometries with indexed triangles.

Changes

• rendering::TriangulatePolygon() converts polygon rings into an indexed TriangleMesh using OGRGeometry::ConstrainedDelaunayTriangulation() (GDAL 3.12, backed by GEOS). No custom triangulation algorithm; invalid input is rejected explicitly and never repaired.
• VectorRenderScene gains fillMeshes: one optional mesh per polygon, aligned by index, prepared once when the scene is created so no renderer triangulates per frame. A polygon whose triangulation fails keeps its rings (for outlines and GDI) and simply has no mesh.
• Meshes deduplicate vertices through the index buffer and have consistent counter-clockwise winding; coordinates stay in the source coordinate system (2D rendering mesh, no Z/M redesign).
• New GoogleTest coverage: convex/concave/holed/multipart meshes, index validity, winding, area sums vs. polygon area, containment, non-overlap, hole exclusion, and input validation.

Verification

• msbuild MapWinGIS6.slnx -restore -p:Configuration=Debug -p:Platform=x64 -m — succeeds with zero warnings.
• MapWinGIS.Core.Tests.exe: 311 passed (20 new triangulation tests), 1 pre-existing skip.
• MapWinGIS.ActiveX.Tests.exe: 56 passed, 2 pre-existing skips.

Memory: recorded GDAL 3.12 snake_case geometry accessors and GeoPackage invalid-geometry fixture behavior in .ai/memory/.

Dependencies

Stacked plan PR 1 of 3. PR 2 (polygon-triangulation-opengl, against this branch) will consume the meshes in the OpenGL backend; PR 3 verifies the pipeline and updates rendering documentation.

[Julian-MW]

One edge-case correctness issue found; otherwise the implementation and test coverage look sound.

[Copilot]

🟡 Changes recommended

Critical exception handling in PolygonTriangulation.cpp discards captured logger exceptions and converts std::bad_alloc into recoverable geometry errors, preventing failures from reaching their intended handlers.

2 open issues

• Preserve std::bad_alloc during triangulation · New
• Rethrow captured exceptions after validation and triangulation · New

What changed in this PR

Adds reusable polygon triangulation to Core, preparing backend-neutral meshes for a later OpenGL update while preserving existing ring-based rendering.

Changes:

• Introduces GEOS-backed triangulation with shared vertices, indexed triangles, and consistent counter-clockwise winding.
• Prepares optional meshes alongside scene polygons, retaining original rings when triangulation fails.
• Adds geometry and scene-integration tests, build registration, and documentation.

Builds and tests were not run during this review.

File	Description
tests/​MapWinGIS.Core.Tests/​PolygonTriangulationTests.cpp	Tests mesh geometry, validation, and scene integration.
tests/​MapWinGIS.Core.Tests/​MapWinGIS.Core.Tests.vcxproj	Registers the triangulation tests.
src/​MapWinGIS.Core/​src/​rendering/​VectorRenderScene.cpp	Prepares optional per-polygon fill meshes.
src/​MapWinGIS.Core/​src/​rendering/​PolygonTriangulation.cpp	Implements validation, triangulation, and indexed mesh assembly.
src/​MapWinGIS.Core/​MapWinGIS.Core.vcxproj	Registers the new Core header and implementation.
src/​MapWinGIS.Core/​include/​mapwingis/​rendering/​VectorRenderScene.h	Defines triangle meshes and aligned scene storage.
src/​MapWinGIS.Core/​include/​mapwingis/​rendering/​PolygonTriangulation.h	Declares the public triangulation API and contract.
docs/​ARCHITECTURE.md	Documents scene-time mesh preparation and ring retention.
.ai/​memory/​testing/​testing.md	Records GeoPackage geometry-fixture behavior.
.ai/​memory/​build/​dependencies.md	Records GDAL accessor and intersection-result notes.

_{Review effort: Balanced}