Unity 3D Procedural Terrain - Step 4 - Multi-Layer Perlin

using UnityEngine;
using System.Collections;

public class TerrainNoise : MonoBehaviour {

    float waterSmooth = 0;
    float[,] heights;
    Terrain terrain;
    TerrainData data;
    Transform water;
    int noiseLayerCount = 20;
    NoiseLayer[] noiseLayers;
    float heightBoost = 10;
    float updateTime;
    bool dirty = true;

    void OnGUI()
    {
        GUILayout.BeginArea( new Rect(20,20,400,400) );

        GUI.color = Color.black;

        GUILayout.Label("WaterSmooth: "+waterSmooth);
        float newWaterSmooth = GUILayout.HorizontalSlider(waterSmooth, 0f, 1f);
        if (newWaterSmooth != waterSmooth)
        {
            waterSmooth = newWaterSmooth;
            dirty = true;
            updateTime = Time.time + 0.5f;
        }

        GUILayout.Label("HeightBoost: "+heightBoost);
        float newHeightBoost = GUILayout.HorizontalSlider(heightBoost, 1f, 40f);
        if (newHeightBoost != heightBoost)
        {
            heightBoost = newHeightBoost;
            dirty = true;
            updateTime = Time.time + 0.5f;
        }

        GUILayout.EndArea();

    }

    void Start()
    {

        terrain = GetComponent<Terrain>();
        data = terrain.terrainData;
        heights = new float[ data.heightmapWidth, data.heightmapHeight];
        water = GameObject.Find ("Water").transform;



        // create the required number of noise layer definitions
        noiseLayers = new NoiseLayer[noiseLayerCount];
        float frequency = .003f;
        float amplitude = 0.5f;
        for( int n=0; n<noiseLayers.Length; ++n)
        {
            noiseLayers[n] = new NoiseLayer();
            noiseLayers[n].frequency = frequency;
            noiseLayers[n].amplitude = amplitude;

            amplitude *= 0.5f;
            frequency *= 2f;

        }

    }

    // Use this for initialization
    void Update () {

        if (dirty && Time.time > updateTime)
        {

            float waterHeight = water.position.y / data.size.y;

            for( int x=0; x< data.heightmapWidth; ++x)
            {
                for( int z=0; z< data.heightmapHeight; ++z)
                {
                    // accumulate height values from each perlin noise layer
                    float height = 0;
                    foreach(NoiseLayer noiseLayer in noiseLayers)
                    {
                        float f = noiseLayer.frequency;
                        float a = noiseLayer.amplitude;
                        height += Mathf.PerlinNoise(x*f, z*f) * a;
                    }

                    // smooth heights at water level
                    float waterSmoothedHeight = height - waterHeight;
                    float heightSign = Mathf.Sign(waterSmoothedHeight);
                    waterSmoothedHeight *= (waterSmoothedHeight * heightSign) * heightBoost;
                    waterSmoothedHeight += waterHeight;


                    height = Mathf.Lerp( height, waterSmoothedHeight, waterSmooth );

                    heights[x,z] = height;

                }
            }

            data.SetHeights(0, 0, heights);


            dirty = false;
        }
    }

}


public struct NoiseLayer
{
    public float amplitude;
    public float frequency;

}