Unity NavMesh 动态烘焙

最初的Unity导航系统很不完善，只能静态烘焙场景图的可行走区域，而且必须在本地保存场景的NavMesh数据，难以运行时动态计算；这使得鲜有开发者愿意再尝试Unity内置的导航功能，转向了AStar寻路算法的研究。

但实际上AStar算法真的适合大多数开发情况且性能较优么？

了解过AStar算法的都知道，它是基于格子来遍历计算行走权重的，算法复杂度其实是相对较高的，受到格子密度，地图大小和路线长度的的影响较大。

AStar更适合的是策略性寻路，该算法更有利于找出最短路径的最优解，能够达到足够的精确性。

而Unity的NavMesh是用的拐角点算法，随便找一个场景烘焙一下便可得知，例如：

image.png

烘焙出来的NavMesh区域只在障碍物边缘与平面边缘存在顶点，而不会像AStar一样均匀的布满整个平面；如果是一个无任何障碍物的平面，那就只会有平面边缘的几个顶点，算法效率是相对较高的，并不会因为地图变大而有明显算法复杂度上的变化。

相反，NavMesh的缺点也正是AStar的优点，那就是难以保证寻路的最优解，更多的时候是用于AI能够更快计算出绕过障碍物朝向目标前进的路径。

对于场景不变的静态地图来说，Unity最初的NavMesh已经能够满足需求，但如果地图随机生成或障碍物的位置随时变化，此时静态NavMesh一下子就捉襟见肘了。

好在随着Unity版本的更新，关于动态烘焙的方法也已经能有效实现，这样无论是以怎样千变万化的方式生成的随机地图，随机地图在游戏中如何构建重组，都能动态刷新出NavMesh的可行走区域。

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类是为了收集需要录入烘焙列表的模型网格数据和地形数据，用的是一个全局的静态数据列表来存储，需要挂载在场景的网格物件上，标记哪些物件的网格在生成数据时需要考虑在内。
当然了，如果一个物体是由多个网格拼接而成，读者只需要将OnEnable和OnDisable中的代码稍作修改，改为读取子物体中的所以MeshFilter和Terrain组件即可：

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

将之前收集到的网格物件的源数据动态烘焙刷新生成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>
    /// 按范围动态更新NavMesh
    /// </summary>
    /// <param name="asyncUpdate">是否异步加载</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);
    }

    //选择物体时在Scene中绘制Bound区域
    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);
    }

    //动态烘焙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);
    }
}

有一个地方需要注意，因为NavMeshBuilder.UpdateNavMeshData(m_NavMesh, defaultBuildSettings, m_Sources, bounds); 刷新NavMeshData时需要读取模型的网格信息，此时需要将导入的模型读写打开，设置位置如下：

image.png

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);
    }
}

例如进入某一房间或区域就按照该房间区域的大小进行NavMesh的动态烘焙，可以非常方便的改变烘焙的范围和中心点等，也可以考虑让该烘焙范围一直跟随玩家的Transform运动。

一个区域内的NavMesh动态烘焙完成后，很多AI可能需要在NavMesh中取随机点进行导航的目标点的设置或巡逻等，可以写一个扩展方法得到NavMesh的顶点数据，取任何一个三角内的点即可：

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;
    }