草地优化

草地编辑器
3.3k words

分块管理(空间数据结构上的优化)

在考虑大场景的情况下，可能存在几百万乃至更多的草，如果每帧都去进行可见性剔除，开销过大。所以选择使用空间加速算法来优化，通过划分多个AABB，根据AABB是否与摄像机裁剪区域相交、包含，再进行具体草的剔除

图形引擎实战：一种大世界巨量植被管理与渲染方案 - 搜狐畅游引擎部的文章 - 知乎

CPU裁剪

对于草的数据构建一个BVH树，使用AABB包围指定数量的草，通过判断AABB是否与摄像机区域交相、包含，决定是否传递数据给GPU
UnityURP-MobileDrawMeshInstancedIndirectExample

作者AABB裁剪的思想

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for (int i = 0; i < cellPosWSsList.Length; i++)
{
    //create cell bound
    Vector3 centerPosWS = new Vector3 (i % cellCountX + 0.5f, 0, i / cellCountX + 0.5f);
    centerPosWS.x = Mathf.Lerp(minX, maxX, centerPosWS.x / cellCountX);
    centerPosWS.z = Mathf.Lerp(minZ, maxZ, centerPosWS.z / cellCountZ);
    Vector3 sizeWS = new Vector3(Mathf.Abs(maxX - minX) / cellCountX,0,Mathf.Abs(maxX - minX) / cellCountX);
    Bounds cellBound = new Bounds(centerPosWS, sizeWS);

    if (GeometryUtility.TestPlanesAABB(cameraFrustumPlanes, cellBound))
    {
        visibleCellIDList.Add(i);
    }
}

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先将所有草的数据传递给ComputeShader
遍历各个 AABB，如果和摄像机裁剪区域相交，则将 AABB 内的草数据传递给 ComputeShader，ComputeShader 中再根据裁剪空间进行裁剪

我AABB裁剪的思想
在绘制时，自己指定AABB的区间，使用顶点色的Alpha通道，根据AABB和摄像机裁剪区域相交情况，将数据传递给 ComputeShader
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细节

剔除后的所有顶点数据
BVH_AABB.cs

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public List<NodeData> nodeDataList = new List<NodeData>();
public List<NodeData> v_nodeDataList
{
    get {
        if (nodeDataList == null || nodeDataList.Count == 0) {
            AddVisibleList(tree);
        }
        return nodeDataList;
    }
}

private void AddVisibleList(BVHNode node) {
    if (node != null)
    {
        if (node.nodeDataList.Count > 0)
        {
            if (GeometryUtility.TestPlanesAABB(cameraFrustumPlanes, node.aabb.bounds))
            {
                visibleAABB.Add(node.aabb);
                nodeDataList.AddRange(node.nodeDataList);
            }
        }
        if (node.left != null) {
            AddVisibleList(node.left);
        }
        if (node.right != null) {
            AddVisibleList(node.right);
        }
    }
}

GPU裁剪

进行裁剪添加数据
Github-HiZ_grass_culling

1. 转换为裁剪空间剔除

Unity Shader 各个空间坐标的获取方式及xyzw含义 - 小电脑的文章 - 知乎
裁剪空间下，裁剪区域内的坐标 -w<=(x/y/z)<=w

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float4 absPosCS = abs(mul(_VPMatrix,float4(_AllInstancesPosWSBuffer[id.x + _StartOffset],1.0)));

//do culling test in clip space, result is the same as doing test in NDC space.
//prefer clip space here because doing culling test in clip space is faster than doing culling test in NDC, because we can skip 1 division.
//the test is using OpenGL standard projection matrix, because all matrix from unity C# is OpenGL standard
//if instance is inside camera frustum, and is within draw distance, we append it to _VisibleInstanceOnlyTransformBuffer
//y test allow 50% more threshold (hardcode for grass)
//x test allow 10% more threshold (hardcode for grass)
if (absPosCS.z <= absPosCS.w && absPosCS.y <= absPosCS.w*1.5 && absPosCS.x <= absPosCS.w*1.1 && absPosCS.w <= _MaxDrawDistance)
    _VisibleInstancesOnlyPosWSIDBuffer.Append(id.x + _StartOffset);

LOD处理

距离远的草没必要使用和近处草同样的顶点

1. 设置距离外的草，不渲染

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float3 worldPos = mul(_LocalToWorld, float4(sv.positionOS, 1)).xyz;
float distanceFromCamera = distance(worldPos, _CameraPositionWS);
float distanceCutoff = saturate(1 - saturate((distanceFromCamera - _MinFadeDist) / (_MaxFadeDist - _MinFadeDist)));

if(numBladesPerVertex <= 0 || distanceCutoff <= 0){
    return;
}

2. 设置宽度的渐变，不会显得突兀

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float distanceFade = saturate(1 - saturate((distanceFromCamera - _MinFadeDist - 5) / (_MaxFadeDist - _MinFadeDist -5)));

_GrassWidth *= (distanceFade);

3. 顶点数的渐变

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float distanceFade = saturate(1 - saturate((distanceFromCamera - _MinFadeDist - 5) / (_MaxFadeDist - _MinFadeDist -5)));

numSegmentsPerBlade = max(1, numSegmentsPerBlade * distanceFade);