Unity NavMesh 動態烘焙繪制與隨機取點

最初的Unity導航系統很不完善，只能靜態烘焙場景圖的可行走區域，而且必須在本地保存場景的NavMesh數據，難以運行時動態計算；這使得鮮有開發者願意再嘗試Unity內置的導航功能，轉向了AStar尋路算法的研究。

但實際上AStar算法真的適合大多數開發情況且性能較優么？

了解過AStar算法的都知道，它是基於格子來遍歷計算行走權重的，算法復雜度其實是相對較高的，受到格子密度，地圖大小和路線長度的的影響較大。

AStar更適合的是策略性尋路，該算法更有利於找出最短路徑的最優解，能夠達到足夠的精確性。

而Unity的NavMesh是用的拐角點算法，隨便找一個場景烘焙一下便可得知，例如：

 

烘焙出來的NavMesh區域只在障礙物邊緣與平面邊緣存在頂點，而不會像AStar一樣均勻的布滿整個平面；如果是一個無任何障礙物的平面，那就只會有平面邊緣的幾個頂點，算法效率是相對較高的，並不會因為地圖變大而有明顯算法復雜度上的變化。

相反，NavMesh的缺點也正是AStar的優點，那就是難以保證尋路的最優解，更多的時候是用於AI能夠更快計算出繞過障礙物朝向目標前進的路徑。

 

對於場景不變的靜態地圖來說，Unity最初的NavMesh已經能夠滿足需求，但如果地圖隨機生成或障礙物的位置隨時變化，此時靜態NavMesh一下子就捉襟見肘了。

好在隨着Unity版本的更新，關於動態烘焙的方法也已經能有效實現，這樣無論是以怎樣千變萬化的方式生成的隨機地圖，隨機地圖在游戲中如何構建重組，都能動態刷新出NavMesh的可行走區域。

using UnityEngine;
using UnityEngine.AI;
using System.Collections.Generic;

// Tagging component for use with the LocalNavMeshBuilder
// Supports mesh-filter and terrain - can be extended to physics and/or primitives
[DefaultExecutionOrder(-200)]
public class NavMeshSourceTag : MonoBehaviour
{
    // Global containers for all active mesh/terrain tags
    public static List<MeshFilter> m_Meshes = new List<MeshFilter>();
    public static List<Terrain> m_Terrains = new List<Terrain>();

    void OnEnable()
    {
        var m = GetComponent<MeshFilter>();
        if (m != null)
        {
            m_Meshes.Add(m);
        }

        var t = GetComponent<Terrain>();
        if (t != null)
        {
            m_Terrains.Add(t);
        }
    }

    void OnDisable()
    {
        var m = GetComponent<MeshFilter>();
        if (m != null)
        {
            m_Meshes.Remove(m);
        }

        var t = GetComponent<Terrain>();
        if (t != null)
        {
            m_Terrains.Remove(t);
        }
    }

    // Collect all the navmesh build sources for enabled objects tagged by this component
    public static void Collect(ref List<NavMeshBuildSource> sources)
    {
        sources.Clear();

        for (var i = 0; i < m_Meshes.Count; ++i)
        {
            var mf = m_Meshes[i];
            if (mf == null) continue;

            var m = mf.sharedMesh;
            if (m == null) continue;

            var s = new NavMeshBuildSource();
            s.shape = NavMeshBuildSourceShape.Mesh;
            s.sourceObject = m;
            s.transform = mf.transform.localToWorldMatrix;
            s.area = 0;
            sources.Add(s);
        }

        for (var i = 0; i < m_Terrains.Count; ++i)
        {
            var t = m_Terrains[i];
            if (t == null) continue;

            var s = new NavMeshBuildSource();
            s.shape = NavMeshBuildSourceShape.Terrain;
            s.sourceObject = t.terrainData;
            // Terrain system only supports translation - so we pass translation only to back-end
            s.transform = Matrix4x4.TRS(t.transform.position, Quaternion.identity, Vector3.one);
            s.area = 0;
            sources.Add(s);
        }
    }
}

NavMeshSourceTag類是為了收集需要錄入烘焙列表的模型網格數據和地形數據，用的是一個全局的靜態數據列表來存儲，需要掛載在場景的網格物件上，標記哪些物件的網格在生成數據時需要考慮在內。
當然了，如果一個物體是由多個網格拼接而成，讀者只需要將OnEnable和OnDisable中的代碼稍作修改，改為讀取子物體中的所以MeshFilter和Terrain組件即可：

        foreach (var m in GetComponentsInChildren<MeshFilter>())
        {
            if (m != null)
            {
                m_Meshes.Add(m);
            }
        }

將之前收集到的網格物件的源數據動態烘焙刷新生成NavMesh：

using UnityEngine;
using UnityEngine.AI;
using System.Collections;
using System.Collections.Generic;
using NavMeshBuilder = UnityEngine.AI.NavMeshBuilder;

// Build and update a localized navmesh from the sources marked by NavMeshSourceTag
[DefaultExecutionOrder(-102)]
public class LocalNavMeshBuilder : MonoBehaviour
{
    // The center of the build
    public Transform m_Tracked;

    // The size of the build bounds
    public Vector3 m_Size = new Vector3(80.0f, 20.0f, 80.0f);

    NavMeshData m_NavMesh;
    AsyncOperation m_Operation;
    NavMeshDataInstance m_Instance;
    List<NavMeshBuildSource> m_Sources = new List<NavMeshBuildSource>();

    IEnumerator Start()
    {
        while (true)
        {
            UpdateNavMesh(true);
            yield return m_Operation;
        }
    }

    void OnEnable()
    {
        Bake();
    }

    void OnDisable()
    {
        //Unload navmesh and clear handle
        m_Instance.Remove();
    }

    /// <summary>
    /// 按范圍動態更新NavMesh
    /// </summary>
    /// <param name="asyncUpdate">是否異步加載</param>
    void UpdateNavMesh(bool asyncUpdate = false)
    {
        NavMeshSourceTag.Collect(ref m_Sources);
        var defaultBuildSettings = NavMesh.GetSettingsByID(0);
        var bounds = QuantizedBounds();

        if (asyncUpdate)
            m_Operation = NavMeshBuilder.UpdateNavMeshDataAsync(m_NavMesh, defaultBuildSettings, m_Sources, bounds);
        else
            NavMeshBuilder.UpdateNavMeshData(m_NavMesh, defaultBuildSettings, m_Sources, bounds);
    }

    static Vector3 Quantize(Vector3 v, Vector3 quant)
    {
        float x = quant.x * Mathf.Floor(v.x / quant.x);
        float y = quant.y * Mathf.Floor(v.y / quant.y);
        float z = quant.z * Mathf.Floor(v.z / quant.z);
        return new Vector3(x, y, z);
    }

    Bounds QuantizedBounds()
    {
        // Quantize the bounds to update only when theres a 0.1% change in size
        var center = m_Tracked ? m_Tracked.position : transform.position;
        return new Bounds(Quantize(center, .001f * m_Size), m_Size);
    }

    //選擇物體時在Scene中繪制Bound區域
    void OnDrawGizmosSelected()
    {
        if (m_NavMesh)
        {
            Gizmos.color = Color.green;
            Gizmos.DrawWireCube(m_NavMesh.sourceBounds.center, m_NavMesh.sourceBounds.size);
        }

        Gizmos.color = Color.yellow;
        var bounds = QuantizedBounds();
        Gizmos.DrawWireCube(bounds.center, bounds.size);

        Gizmos.color = Color.green;
        var center = m_Tracked ? m_Tracked.position : transform.position;
        Gizmos.DrawWireCube(center, m_Size);
    }

    //動態烘焙NavMesh
    public void Bake()
    {
        // Construct and add navmesh
        m_NavMesh = new NavMeshData();
        m_Instance = NavMesh.AddNavMeshData(m_NavMesh);
        if (m_Tracked == null)
            m_Tracked = transform;
        UpdateNavMesh(false);
    }
}

有一個地方需要注意，因為NavMeshBuilder.UpdateNavMeshData(m_NavMesh, defaultBuildSettings, m_Sources, bounds); 刷新NavMeshData時需要讀取模型的網格信息，此時需要將導入的模型讀寫打開，設置位置如下：

用法示例：

using UnityEngine;

public class LocalNavMeshCtrl : MonoBehaviour
{
    public LocalNavMeshBuilder Bulider;
    public float Offse;
    void Awake()
    {
        EventManager.AddListener<EnterRoomEvent>(EnterRoomHanlder);
    }

    private void EnterRoomHanlder(EnterRoomEvent e)
    {
        if (Bulider != null)
        {
            var rooms = BattleUtils.MapMgr.Rooms;
            if (rooms.ContainsKey(e.RoomIndex) && rooms[e.RoomIndex].RoomType == RoomType.Battle)
            {
                Bulider.m_Tracked = rooms[e.RoomIndex].transform;
                var size = PTBattleMgr.CurRoomCtrl.Size;
                Bulider.m_Size = new Vector3(size.x * 4 + Offse, 10, size.y * 4 + Offse);
            }
        }
    }

    private void OnDestroy()
    {
        EventManager.RemoveListener<EnterRoomEvent>(EnterRoomHanlder);
    }
}

例如進入某一房間或區域就按照該房間區域的大小進行NavMesh的動態烘焙，可以非常方便的改變烘焙的范圍和中心點等，也可以考慮讓該烘焙范圍一直跟隨玩家的Transform運動。

 

一個區域內的NavMesh動態烘焙完成后，很多AI可能需要在NavMesh中取隨機點進行導航的目標點的設置或巡邏等，可以寫一個擴展方法得到NavMesh的頂點數據，取任何一個三角內的點即可：

    public static Vector3 GetNavMeshRandomPos(this GameObject obj)
    {
        NavMeshTriangulation navMeshData = NavMesh.CalculateTriangulation();

        int t = Random.Range(0, navMeshData.indices.Length - 3);

        Vector3 point = Vector3.Lerp(navMeshData.vertices[navMeshData.indices[t]], navMeshData.vertices[navMeshData.indices[t + 1]], Random.value);
        point = Vector3.Lerp(point, navMeshData.vertices[navMeshData.indices[t + 2]], Random.value);

        return point;
    }