Unity剔除优化原理

发布时间: 2025-03-23 09:02

↑

# Unity剔除优化原理

本文将深入探讨Unity的剔除优化系统原理。通过本文,你将了解Unity中各种剔除技术的工作机制、性能特点和最佳实践,以及如何利用这些技术来优化游戏渲染性能。

## 基础概念

### 什么是剔除

剔除(Culling)是一种渲染优化技术,它可以:

1. 减少渲染对象
2. 降低渲染开销
3. 提高渲染效率
4. 优化内存使用

### 剔除类型

```csharp
// 剔除系统示例
public class CullingSystem : MonoBehaviour
{
    private void Update()
    {
        // 1. 视锥体剔除
        if (!GeometryUtility.TestPlanesAABB(
            GeometryUtility.CalculateFrustumPlanes(Camera.main),
            renderer.bounds))
            return;

// 2. 遮挡剔除
        if (Physics.Raycast(
            Camera.main.transform.position,
            transform.position - Camera.main.transform.position,
            out RaycastHit hit))
        {
            if (hit.collider.gameObject != gameObject)
                return;
        }

// 3. 距离剔除
        float distance = Vector3.Distance(
            Camera.main.transform.position,
            transform.position);
        if (distance > cullingDistance)
            return;

// 渲染对象
        renderer.enabled = true;
    }
}
```

主要类型:

1. 视锥体剔除
   - 剔除视野外对象
   - 基于包围盒
   - 快速高效

2. 遮挡剔除
   - 剔除被遮挡对象
   - 基于深度测试
   - 减少过度绘制

3. 距离剔除
   - 剔除远距离对象
   - 基于LOD系统
   - 优化渲染负载

## 视锥体剔除

### 工作原理

```csharp
// 视锥体剔除实现
public class FrustumCulling
{
    private struct Plane
    {
        public Vector3 normal;
        public float distance;

public float GetDistanceToPoint(Vector3 point)
        {
            return Vector3.Dot(normal, point) + distance;
        }
    }

private class Frustum
    {
        private Plane[] planes = new Plane[6]; // 近、远、左、右、上、下

public void CalculatePlanes(Camera camera)
        {
            Matrix4x4 matrix = camera.projectionMatrix * camera.worldToCameraMatrix;

// 提取视锥体平面
            planes[0] = ExtractPlane(matrix, 0); // 左
            planes[1] = ExtractPlane(matrix, 1); // 右
            planes[2] = ExtractPlane(matrix, 2); // 下
            planes[3] = ExtractPlane(matrix, 3); // 上
            planes[4] = ExtractPlane(matrix, 4); // 近
            planes[5] = ExtractPlane(matrix, 5); // 远
        }

private Plane ExtractPlane(Matrix4x4 matrix, int row)
        {
            Plane plane;
            plane.normal.x = matrix[row, 0];
            plane.normal.y = matrix[row, 1];
            plane.normal.z = matrix[row, 2];
            plane.distance = matrix[row, 3];
            return plane;
        }

public bool TestSphere(Vector3 center, float radius)
        {
            foreach (var plane in planes)
            {
                if (plane.GetDistanceToPoint(center) < -radius)
                    return false;
            }
            return true;
        }
    }
}
```

关键步骤:

1. 视锥体构建
   - 提取平面方程
   - 计算法线方向
   - 优化平面表示

2. 相交测试
   - 包围盒测试
   - 球体测试
   - 快速剔除

3. 空间优化
   - 八叉树
   - BSP树
   - 场景划分

### 性能优化

```csharp
// 视锥体剔除优化
public class FrustumCullingOptimization
{
    private class OctreeNode
    {
        public Bounds bounds;
        public List<Renderer> objects;
        public OctreeNode[] children;

public void Subdivide()
        {
            children = new OctreeNode[8];
            Vector3 size = bounds.size * 0.5f;
            Vector3 center = bounds.center;

// 创建子节点
            for (int i = 0; i < 8; i++)
            {
                Vector3 offset = new Vector3(
                    ((i & 1) != 0 ? size.x : -size.x),
                    ((i & 2) != 0 ? size.y : -size.y),
                    ((i & 4) != 0 ? size.z : -size.z));

children[i] = new OctreeNode
                {
                    bounds = new Bounds(
                        center + offset * 0.5f,
                        size)
                };
            }

// 分配对象到子节点
            foreach (var obj in objects)
            {
                for (int i = 0; i < 8; i++)
                {
                    if (children[i].bounds.Intersects(obj.bounds))
                    {
                        children[i].objects.Add(obj);
                    }
                }
            }
        }

public void Cull(Plane[] planes, List<Renderer> visible)
        {
            // 检查节点是否在视锥体内
            if (!GeometryUtility.TestPlanesAABB(planes, bounds))
                return;

// 如果是叶子节点,添加可见对象
            if (children == null)
            {
                visible.AddRange(objects);
                return;
            }

// 递归处理子节点
            for (int i = 0; i < 8; i++)
            {
                if (children[i].objects.Count > 0)
                    children[i].Cull(planes, visible);
            }
        }
    }
}
```

优化策略:

1. 空间划分
   - 八叉树
   - 四叉树
   - 网格划分

2. 层次剔除
   - 自顶向下
   - 包围体测试
   - 早期剔除

3. SIMD优化
   - 向量运算
   - 批量测试
   - 并行处理

## 遮挡剔除

### 工作原理

```csharp
// 遮挡剔除实现
public class OcclusionCulling
{
    private struct OccluderData
    {
        public Mesh mesh;
        public Matrix4x4 transform;
        public float importance;
    }

private class OcclusionSystem
    {
        private List<OccluderData> occluders;
        private RenderTexture depthBuffer;

public void Initialize(Camera camera)
        {
            // 创建深度缓冲
            depthBuffer = new RenderTexture(
                camera.pixelWidth / 4,
                camera.pixelHeight / 4,
                24,
                RenderTextureFormat.Depth);

// 初始化遮挡器列表
            occluders = new List<OccluderData>();
        }

public void UpdateOccluders(Camera camera)
        {
            // 渲染遮挡器到深度缓冲
            Graphics.SetRenderTarget(depthBuffer);
            GL.Clear(true, true, Color.black);

foreach (var occluder in occluders)
            {
                Graphics.DrawMesh(
                    occluder.mesh,
                    occluder.transform,
                    occluderMaterial,
                    0);
            }

// 使用深度缓冲进行遮挡测试
            Shader.SetGlobalTexture("_OcclusionDepth", depthBuffer);
        }

public bool IsVisible(Bounds bounds, Camera camera)
        {
            // 转换包围盒到屏幕空间
            Vector3[] corners = GetBoundsCorners(bounds);
            foreach (var corner in corners)
            {
                Vector4 clipPos = camera.projectionMatrix *
                                 camera.worldToCameraMatrix *
                                 new Vector4(corner.x, corner.y, corner.z, 1);

// 深度比较
                if (clipPos.z/clipPos.w < GetStoredDepth(clipPos))
                    return true;
            }
            return false;
        }
    }
}
```

实现细节:

1. 深度缓冲
   - 降采样优化
   - 分层渲染
   - 深度压缩

2. 遮挡测试
   - 包围盒测试
   - 分层测试
   - 保守测试

3. 性能优化
   - 异步更新
   - 时间分布
   - 缓存结果

### 优化策略

```csharp
// 遮挡剔除优化
public class OcclusionOptimization
{
    private class HierarchicalDepthBuffer
    {
        private RenderTexture[] depthLevels;
        private int levelCount;

public void Initialize(int width, int height)
        {
            // 创建分层深度缓冲
            levelCount = Mathf.CeilToInt(Mathf.Log(Mathf.Max(width, height), 2));
            depthLevels = new RenderTexture[levelCount];

for (int i = 0; i < levelCount; i++)
            {
                depthLevels[i] = new RenderTexture(
                    width >> i,
                    height >> i,
                    24,
                    RenderTextureFormat.RFloat);
            }
        }

public void UpdateHierarchy()
        {
            // 从细到粗更新深度层级
            for (int i = 1; i < levelCount; i++)
            {
                Graphics.Blit(
                    depthLevels[i-1],
                    depthLevels[i],
                    downsampleMaterial);
            }
        }

public bool TestVisibility(
            Bounds bounds,
            Camera camera,
            int level)
        {
            // 使用适当层级进行测试
            Vector2 screenSize = new Vector2(
                depthLevels[level].width,
                depthLevels[level].height);

Rect screenRect = GetScreenRect(bounds, camera);
            Vector2 pixelRect = screenRect.size * screenSize;

// 如果包围盒太小,使用更细的层级
            if (pixelRect.x < 2 || pixelRect.y < 2)
                return TestVisibility(bounds, camera, level - 1);

// 执行遮挡测试
            return !IsFullyOccluded(
                screenRect,
                depthLevels[level]);
        }
    }
}
```

优化方向:

1. 分层处理
   - 多级深度图
   - 自适应采样
   - 渐进更新

2. 时间分布
   - 异步更新
   - 帧间分布
   - 优先级排序

3. 硬件加速
   - GPU查询
   - 硬件遮挡
   - 异步读回

元素码农