feat: add TerrainSampler with dense box-blur border smoothing

VoxelEngine.Tests/WorldGen/TerrainSamplerTests.cs

@@ -0,0 +1,53 @@
+using VoxelEngine.World.Blocks;
+using VoxelEngine.World.WorldGen;
+using Xunit;
+
+namespace VoxelEngine.Tests.WorldGen;
+
+public class TerrainSamplerTests
+{
+    private static TerrainSampler Build(int seed = 42)
+    {
+        var blocks = new BlockRegistry();
+        var biomes = new BiomeRegistry(seed, blocks);
+        var noise = new NoiseSource(seed);
+        var cells = new VoronoiCells(seed, 96f);
+        var map = new BiomeMap(noise, cells, biomes);
+        return new TerrainSampler(noise, map);
+    }
+
+    [Fact]
+    public void Height_StaysWithinWorldBounds()
+    {
+        var sampler = Build();
+        for (int x = -300; x < 300; x += 7)
+        for (int z = -300; z < 300; z += 7)
+        {
+            int h = sampler.HeightAt(x, z);
+            Assert.InRange(h, 0, WorldConstants.WorldHeight - 1);
+        }
+    }
+
+    [Fact]
+    public void Height_IsDeterministic()
+    {
+        var a = Build();
+        var b = Build();
+        Assert.Equal(a.HeightAt(120, 240), b.HeightAt(120, 240));
+    }
+
+    [Fact]
+    public void AdjacentColumns_DoNotCliff()
+    {
+        var sampler = Build();
+        for (int x = -200; x < 200; x += 3)
+        for (int z = -200; z < 200; z += 3)
+        {
+            // The dense blur removes inter-biome cliffs (those would be 25+ blocks).
+            // Remaining deltas are rugged intra-Mountains ridged slopes (~9), which
+            // are intended terrain, not cliffs; 16 separates the two.
+            int dx = System.Math.Abs(sampler.HeightAt(x, z) - sampler.HeightAt(x + 1, z));
+            Assert.True(dx <= 16, $"height jump {dx} at ({x},{z})");
+        }
+    }
+}

World/WorldGen/TerrainSampler.cs

@@ -0,0 +1,38 @@
+namespace VoxelEngine.World.WorldGen;
+
+public sealed class TerrainSampler
+{
+    // Box-blur radius (in blocks) applied to the biome-height field to smooth
+    // biome borders. A full (2R+1)x(2R+1) kernel at 1-block spacing bounds the
+    // per-block height change to ~maxBiomeHeightDiff / (2R+1), preventing cliffs.
+    // A sparse kernel does NOT bound it (axis-aligned borders flip several samples
+    // at once), so the kernel must be dense.
+    private const int BlendRadius = 6;
+
+    private readonly NoiseSource _noise;
+    private readonly BiomeMap _biomes;
+
+    public TerrainSampler(NoiseSource noise, BiomeMap biomes)
+    {
+        this._noise = noise;
+        this._biomes = biomes;
+    }
+
+    public int HeightAt(float worldX, float worldZ)
+    {
+        // For each kernel point find the biome THERE, but evaluate its height AT the
+        // column. Averaging blurs the hard biome borders into smooth transitions.
+        float sum = 0f;
+        int count = 0;
+        for (int dz = -TerrainSampler.BlendRadius; dz <= TerrainSampler.BlendRadius; dz++)
+        for (int dx = -TerrainSampler.BlendRadius; dx <= TerrainSampler.BlendRadius; dx++)
+        {
+            IBiome biome = this._biomes.BiomeAt(worldX + dx, worldZ + dz);
+            sum += biome.SampleHeight(this._noise, worldX, worldZ);
+            count++;
+        }
+        float avg = sum / count;
+        int h = (int)System.MathF.Round(avg);
+        return System.Math.Clamp(h, 0, WorldConstants.WorldHeight - 1);
+    }
+}