Worldgen Phase 2: keep LOD biome selection consistent with LOD0

[MichaelFisher1997]

Parent: #646

Phase

Phase 2: Multi-parameter biome selection

Goal

Update LOD biome/height sampling so distant terrain remains visually consistent with full-resolution chunk generation after selector changes.

Tasks

• Review generateHeightmapOnly and LOD representative column sampling.
• Ensure simplified biome selection uses compatible parameter logic.
• Add or update tests/baselines for LOD biome/color output.
• Watch for biome/color seams around mountains, coasts, and rivers.

Acceptance Criteria

• LOD output uses the same core climate/terrain signals as LOD0 where practical.
• No obvious LOD biome discontinuities are introduced by selector changes.
• Existing LOD tests pass.

Dependencies

• Requires multi-parameter selector shape.

Parallelization

Can run in parallel with biome migration and priority-rule preservation.

[github-actions]

Based on my codebase exploration, here's my analysis:

Classification: Feature Request

Analysis

Root Cause / Current State:

The issue stems from a fundamental mismatch between LOD0 and LOD1+ biome selection:

• LOD0 uses selectBiomeWithConstraintsAndRiver() (line ~802 in overworld_generator.zig) - a Voronoi-based selector considering temperature, humidity, height, slope, continentalness, ridge_mask, and river constraints
• LOD1+ uses selectBiomeSimple() (lines 251-289 in biome_selector.zig) - only considers temperature (as heat bands), humidity (high/low), and continentalness (< 0.35 = ocean)

The sampleRepresentativeLODColumn() function (lines 310-385) samples a 3x3 grid and calls classifyLODSample() which ultimately uses dominantBiome() to pick the most frequent biome - but the underlying selection logic is incompatible.

Key Files:

• modules/world-worldgen/src/biome_selector.zig:251-289 - selectBiomeSimple() (simplified selector)
• modules/world-worldgen/src/overworld_generator.zig:310-385 - sampleRepresentativeLODColumn()
• modules/world-worldgen/src/biome_source.zig:116-120 - selectBiomeSimplified() wrapper

Risks / Recommendations

1. Dependency Issue: The "multi-parameter selector shape" referenced in issue Worldgen: biome terrain modifiers and multi-parameter biome selection #646 does not yet exist in the codebase. Phase 2 cannot be fully completed until that foundational work is done.

2. Scope: This task involves multiple subsystems (worldgen + world-lod) and touches biome selection, color output, and test baselines. Consider splitting into:

   • Sub-task A: Audit sampleRepresentativeLODColumn() and identify which parameters are missing from simplified selection
   • Sub-task B: Update selectBiomeSimple() to use compatible signals (or create selectBiomeLOD())
   • Sub-task C: Add test baselines for LOD biome/color consistency
   • Sub-task D: Visual verification for seam artifacts

3. Testing Gap: There are no dedicated tests for LOD biome output consistency. Tests exist in biome_selector_tests.zig and lod_manager_tests.zig but don't validate LOD vs LOD0 biome alignment.

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