VoxelEngine/World/WorldGen/BiomeMap.cs

using VoxelEngine.Core;

namespace VoxelEngine.World.WorldGen;

public sealed class BiomeMap
{
    private readonly NoiseSource _noise;
    private readonly VoronoiCells _cells;
    private readonly BiomeRegistry _biomes;
    private readonly Configuration _cfg;

    public BiomeMap(NoiseSource noise, VoronoiCells cells, BiomeRegistry biomes, Configuration cfg)
    {
        this._noise = noise;
        this._cells = cells;
        this._biomes = biomes;
        this._cfg = cfg;
    }

    public int CellId(float worldX, float worldZ) => this._cells.Nearest(worldX, worldZ).Id;

    public IBiome BiomeAt(float worldX, float worldZ)
        => this.BiomeForCell(this._cells.Nearest(worldX, worldZ));

    // Climate -> biome, sampled at the cell centre so the biome is constant
    // across the cell.
    public IBiome BiomeForCell(VoronoiCells.Cell cell)
    {
        float temperature = this._noise.Temperature(cell.CenterX, cell.CenterZ);
        float elevation = this._noise.Continent(cell.CenterX, cell.CenterZ);
        float forest = this._noise.Forest(cell.CenterX, cell.CenterZ);
        return Classify(temperature, elevation, forest, this._biomes, this._cfg);
    }

    // The biomes of the two nearest cells plus how far inside the nearer cell the
    // point sits (SecondDist - FirstDist, in blocks; 0 exactly on the border).
    // TerrainSampler uses this to ease height across biome borders.
    public readonly record struct BiomeBlend(IBiome Near, IBiome Far, float BorderDistance);

    public BiomeBlend NearestTwo(float worldX, float worldZ)
    {
        VoronoiCells.NearestPair p = this._cells.NearestTwo(worldX, worldZ);
        return new BiomeBlend(
            this.BiomeForCell(p.First),
            this.BiomeForCell(p.Second),
            p.SecondDist - p.FirstDist);
    }

    // Pure climate -> biome lookup. Thresholds in noise units (~[-1,1]).
    public static IBiome Classify(float temperature, float elevation, float forest, BiomeRegistry biomes, Configuration cfg)
    {
        if (elevation > cfg.MountainsElevationThreshold) return biomes.Mountains;          // high ground
        if (temperature > cfg.DesertTemperatureThreshold) return biomes.Desert;            // hot, low
        if (temperature > cfg.ForestTemperatureThreshold && forest > cfg.ForestDensityThreshold) return biomes.Forest; // temperate, wooded
        return biomes.Plains;                                    // temperate/cold, low
    }
}