Description

This PR breaks tile selection algorithms out of private methods in the Tileset class and into free functions. The purpose is to reduce the surface area of Tileset and make the selection algorithms independently testable.

Changes

1) — tileStateUpdater callback: decouple traversal from loading

The traversal helpers (_visitTileIfNeeded, etc.) previously called _pTilesetContentManager->updateTileContent() directly, creating a compile-time dependency on TilesetContentManager inside the selection algorithm. A std::function<void(Tile&)> tileStateUpdater field was added to TilesetFrameState. Tileset::updateViewGroup injects the implementation via a lambda; traversal calls the callback. The traversal has no #include dependency on TilesetContentManager.

Files: TilesetFrameState.h, Tileset.cpp

2) — selectTiles(): pure free function for tile LOD selection

The entire traversal algorithm (_visitTileIfNeeded, _visitTile, _renderLeaf, _renderInnerTile, _visitVisibleChildrenNearToFar, _kickDescendantsAndRenderTile, _loadAndRenderAdditiveRefinedTile, frustum/fog cull, occlusion check, SSE computation) was extracted from Tileset private methods into free functions in a new translation unit TilesetSelection.cpp. The public entry point is:

selectTiles(
    TileSelectionContext& context,   // options + externals + scratch buffers
    TilesetFrameState& frameState,
    Tile& rootTile,
    ViewUpdateResult& result,
);

Tileset::updateViewGroup is now a thin coordinator: it builds frameState and calls selectTiles(). The private traversal method declarations and TraversalDetails/CullResult structs are gone from Tileset.h.

Files: TilesetSelection.h (new), TilesetSelection.cpp (new), Tileset.h, Tileset.cpp

Issue number or link

N/A.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Testing plan

Regression: All existing tests in TestTilesetSelectionAlgorithm.cpp exercise the full selection pipeline via Tileset::updateViewGroup, which now delegates to selectTiles(). These cover: replace/additive refinement, SSE thresholds, frustum/fog culling, multi-frustum, forbidHoles, LOD transition fade-out, unconditionallyRefine.

New isolation test (TestTilesetSelection.cpp):

• selectTiles is callable as a free function — constructs a minimal EllipsoidTilesetLoader-backed tileset, calls selectTiles() directly (bypassing Tileset::updateViewGroup), and asserts at least one tile is selected with no tiles kicked.
• selectTiles result matches updateViewGroup result — runs both updateViewGroup and selectTiles() on the same warmed-up tileset and asserts tile render count and visit count are identical between the two paths.

Build verification: cmake --build build --target Cesium3DTilesSelection succeeds with zero errors.

Remaining Tasks

Things that could be done:

• TilesetViewGroup::startNewFrame/finishFrame still take const Tileset&, which prevents fully async-free unit tests of selectTiles. Decoupling that is a natural follow-up once this PR lands.