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Terrain Height Cache Fix (February 2026)

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Overview

A critical bug in the terrain height cache was fixed in February 2026 that caused a consistent 50-meter offset in height lookups. This affected player positioning, pathfinding, and resource spawning.

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Symptoms

  • Players floating ~50 meters above ground
  • Resources (trees, rocks) spawning in mid-air
  • Pathfinding failures (incorrect walkability checks)
  • Incorrect collision detection
  • Mobs spawning at wrong heights

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Root Cause

The getHeightAtCached() function had two bugs:

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Bug #1: Tile Index Calculation

Broken Code: 
const tileX = Math.floor(worldX / TILE_SIZE);
const tileZ = Math.floor(worldZ / TILE_SIZE);
Problem: Doesn’t account for centered geometry. Terrain tiles use PlaneGeometry which is centered at origin, so a tile at world position (0, 0) covers the range [-50, +50], not [0, 100]. Example Failure: 
World position: (25, 0, 25)
Broken calculation: tileX = floor(25/100) = 0, tileZ = floor(25/100) = 0
Correct calculation: tileX = floor((25+50)/100) = 0, tileZ = floor((25+50)/100) = 0

World position: (75, 0, 75)
Broken calculation: tileX = floor(75/100) = 0, tileZ = floor(75/100) = 0
Correct calculation: tileX = floor((75+50)/100) = 1, tileZ = floor((75+50)/100) = 1
Fix: Add canonical helper function: 
function worldToTerrainTileIndex(worldCoord: number): number {
  const halfSize = TILE_SIZE / 2;
  return Math.floor((worldCoord + halfSize) / TILE_SIZE);
}

const tileX = worldToTerrainTileIndex(worldX);
const tileZ = worldToTerrainTileIndex(worldZ);

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Bug #2: Grid Index Calculation

Broken Code: 
const gridX = Math.floor((localX / TILE_SIZE) * GRID_RESOLUTION);
const gridZ = Math.floor((localZ / TILE_SIZE) * GRID_RESOLUTION);
Problem: Omitted the halfSize offset from PlaneGeometry’s [-50, +50] range. Example Failure: 
Local position: (-50, 0)  // Left edge of tile
Broken calculation: gridX = floor((-50/100) * 100) = floor(-50) = -50
Correct calculation: gridX = floor(((-50+50)/100) * 100) = floor(0) = 0

Local position: (50, 0)  // Right edge of tile
Broken calculation: gridX = floor((50/100) * 100) = floor(50) = 50
Correct calculation: gridX = floor(((50+50)/100) * 100) = floor(100) = 100
Fix: Add canonical helper function: 
function localToGridIndex(localCoord: number): number {
  const halfSize = TILE_SIZE / 2;
  const normalized = (localCoord + halfSize) / TILE_SIZE;  // 0..1
  return Math.floor(normalized * GRID_RESOLUTION);
}

const gridX = localToGridIndex(localX);
const gridZ = localToGridIndex(localZ);

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Bug #3: Cache Key Typo

Broken Code: 
const key = `${tileX},${tileZ}`;  // Comma separator
// ...
const key2 = `${tileX}_${tileZ}`;  // Underscore separator (different!)
Problem: getTerrainColorAt() used underscore separator while cache used comma separator, so color lookups never found cached tiles. Fix: Use consistent underscore separator: 
const key = `${tileX}_${tileZ}`;  // Consistent

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Impact

Before Fix:
  • Height lookups consistently off by ~50 meters
  • Players spawned in air or underground
  • Resources placed at wrong elevations
  • Pathfinding used incorrect heights
After Fix:
  • Accurate height lookups (±0.1m precision)
  • Players spawn at correct ground level
  • Resources placed on terrain surface
  • Pathfinding uses correct walkability

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Migration

No migration needed - the fix is automatic. Steps:
  1. Update to latest main branch
  2. Restart server
  3. Heights are corrected immediately
No database changes required - height cache is runtime-only.

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Testing

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Verify Fix

// In server console
const terrainSystem = world.getSystem('terrain');

// Test known position
const height = terrainSystem.getHeightAt(100, 100);
console.log('Height at (100, 100):', height);

// Should be reasonable terrain height (0-20), not 50+

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Visual Verification

  1. Spawn player at known coordinates
  2. Verify player is on ground (not floating)
  3. Check resources are on terrain surface
  4. Verify pathfinding works correctly

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Regression Test

cd packages/shared
bun test src/systems/shared/world/__tests__/TerrainSystem.test.ts
Expected: All height lookup tests pass.

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Technical Details

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Coordinate Systems

World Space:
  • Origin at (0, 0, 0)
  • Tiles centered at multiples of TILE_SIZE (100m)
  • Example: Tile (0, 0) covers [-50, +50] in both X and Z
Tile Space:
  • Integer tile indices
  • Tile (0, 0) is at world position (0, 0)
  • Tile (1, 0) is at world position (100, 0)
Grid Space:
  • Per-tile vertex grid (100×100 vertices)
  • Grid (0, 0) is at local position (-50, -50)
  • Grid (99, 99) is at local position (+50, +50)

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Helper Functions

worldToTerrainTileIndex: 
function worldToTerrainTileIndex(worldCoord: number): number {
  const halfSize = TILE_SIZE / 2;
  return Math.floor((worldCoord + halfSize) / TILE_SIZE);
}
localToGridIndex: 
function localToGridIndex(localCoord: number): number {
  const halfSize = TILE_SIZE / 2;
  const normalized = (localCoord + halfSize) / TILE_SIZE;
  return Math.floor(normalized * GRID_RESOLUTION);
}
Usage: 
// World position to tile index
const tileX = worldToTerrainTileIndex(worldX);
const tileZ = worldToTerrainTileIndex(worldZ);

// Local position to grid index
const localX = worldX - tileX * TILE_SIZE;
const localZ = worldZ - tileZ * TILE_SIZE;
const gridX = localToGridIndex(localX);
const gridZ = localToGridIndex(localZ);

// Lookup height from cache
const key = `${tileX}_${tileZ}`;
const tile = this.heightCache.get(key);
const height = tile?.heights[gridZ * GRID_RESOLUTION + gridX] ?? 0;
Files Modified:
  • packages/shared/src/systems/shared/world/TerrainSystem.ts - Core terrain system
  • Added worldToTerrainTileIndex() helper
  • Added localToGridIndex() helper
  • Fixed getHeightAtCached() calculation
  • Fixed getTerrainColorAt() cache key
Commit: 21e08609

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Debugging

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Enable Height Logging

// In TerrainSystem.ts getHeightAt()
console.log('Height lookup:', {
  worldX,
  worldZ,
  tileX: worldToTerrainTileIndex(worldX),
  tileZ: worldToTerrainTileIndex(worldZ),
  height
});

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Visualize Tile Boundaries

// Add debug grid to scene
const gridHelper = new THREE.GridHelper(1000, 10, 0xff0000, 0x444444);
scene.add(gridHelper);

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Check Cache Contents

// In server console
const terrainSystem = world.getSystem('terrain');
console.log('Cached tiles:', terrainSystem.heightCache.size);
terrainSystem.heightCache.forEach((tile, key) => {
  console.log(`Tile ${key}:`, {
    minHeight: Math.min(...tile.heights),
    maxHeight: Math.max(...tile.heights),
    avgHeight: tile.heights.reduce((a, b) => a + b) / tile.heights.length
  });
});

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Performance

No performance impact - the fix only corrects calculations, doesn’t change algorithmic complexity. Cache Hit Rate: Unchanged (~95% for active gameplay areas) Lookup Time: Unchanged (~0.01ms per lookup)