private BVHNode BuildNodeRecursively(NodeSpec spec, int depth)
{
// 节点只有一个图元的时候没必要再继续分割
if (spec.numRef <= MIN_LEAF_SIZE || depth >= MAX_DEPTH)
{
return(CreatLeaf(spec));
}
// 挑选使用object split还是spatial split
float leafSAH = spec.bounds.Area * spec.numRef;
float nodeSAH = spec.bounds.Area * 0.125f;//spec.Bounds.Area * 2; // 节点遍历的固定开销，2是个经验值（不一定是最好的）
ObjectSplit objectSplit = FindObjectSplit(spec, nodeSAH);
SpatialSplit spatialSplit = SpatialSplit.New();
if (depth < MAX_SPATIAL_DEPTH)
{
var overlap = objectSplit.leftBounds;
overlap.Intersect(objectSplit.rightBounds);
if (overlap.Area >= _minOverlap)
{
spatialSplit = FindSpatialSplit(spec, nodeSAH);
}
}
// 叶节点胜出，不论是Object还是Spatial slpit，分割后的
float minSAH = Mathf.Min(Mathf.Min(leafSAH, objectSplit.sah), spatialSplit.sah);
if (minSAH == leafSAH && spec.numRef <= MAX_LEAF_SIZE)
{
return(CreatLeaf(spec));
}
// spatial split胜出，尝试执行spatial split
NodeSpec left = NodeSpec.New();
NodeSpec right = NodeSpec.New();
if (minSAH == spatialSplit.sah)
{
PerformSpatialSplit(ref left, ref right, spec, spatialSplit);
}
// objcet split胜出，或spatial split并未取得实质性进展，执行object split
if (left.numRef == 0 || right.numRef == 0)
{
PerformObjectSplit(ref left, ref right, spec, objectSplit);
}
_numDuplicates += left.numRef + right.numRef - spec.numRef;
// 由于后文取下标的方式，一定是先右后左
var rightNode = BuildNodeRecursively(right, depth + 1);
var leftNode = BuildNodeRecursively(left, depth + 1);
return(new InnerNode(spec.bounds, leftNode, rightNode));
}
private SpatialSplit FindSpatialSplit(NodeSpec spec, float nodeSAH)
{
// _bins变量每一次分割都被复用
var origin = spec.bounds.min;
var binSize = (spec.bounds.max - origin) / N_SPATIAL_BINS;
var invBinSize = new Vector3(1f / binSize.x, 1f / binSize.y, 1f / binSize.z);
for (int dim = 0; dim < 3; dim++)
{
for (int i = 0; i < N_SPATIAL_BINS; i++)
{
_bins[dim, i] = SpatialBin.New();
}
}
// 把图元分配到3个维度的bin中
for (int refIdx = _refStack.Count - spec.numRef; refIdx < _refStack.Count; refIdx++)
{
var pRef = _refStack[refIdx];
// ....Vector3Int.FloorToInt 误用了 celling...查半天。。。。
var firstBin = MathUtils.ClampV3Int(Vector3Int.FloorToInt((pRef.bounds.min - origin).Multiply(invBinSize)), Vector3Int.zero, new Vector3Int(N_SPATIAL_BINS - 1, N_SPATIAL_BINS - 1, N_SPATIAL_BINS - 1));
var lastBin = MathUtils.ClampV3Int(Vector3Int.FloorToInt((pRef.bounds.max - origin).Multiply(invBinSize)), firstBin, new Vector3Int(N_SPATIAL_BINS - 1, N_SPATIAL_BINS - 1, N_SPATIAL_BINS - 1));
for (int dim = 0; dim < 3; dim++)
{
var curRef = pRef;
// 从左到右分割,curRef并不更新图元索引，只更新包围盒
for (int i = firstBin[dim]; i < lastBin[dim]; i++)
{
PrimitiveRef leftRef, rightRef;
SplitReference(out leftRef, out rightRef, curRef, dim, origin[dim] + binSize[dim] * (i + 1));
_bins[dim, i].bounds.Union(leftRef.bounds);
curRef = rightRef;
}
_bins[dim, lastBin[dim]].bounds.Union(curRef.bounds); // 分割后图元最右边的包围盒也算进来
// 只对分割后图元所在的第一个和最后一个bin添加图元引用计数
_bins[dim, firstBin[dim]].enter++;
_bins[dim, lastBin[dim]].exit++;
}
}
// 根据分割好的bins，来选择最佳分割平面,跟FindObjectSplit类似,只不过是以bin为单位，而不是图元
SpatialSplit split = SpatialSplit.New();
for (byte dim = 0; dim < 3; dim++)
{
// 从右到左，记录每一种可能的分割后，处在“右边”包围盒的
AABB rightBounds = AABB.New();
for (int i = N_SPATIAL_BINS - 1; i > 0; i--)
{
rightBounds.Union(_bins[dim, i].bounds);
_rightBounds[i - 1] = rightBounds; //_rightBounds用来临时记录右边包围盒的，被复用
}
AABB leftBounds = AABB.New();
int leftNum = 0;
int rightNum = spec.numRef;
for (int i = 1; i < N_SPATIAL_BINS; i++)
{
leftBounds.Union(_bins[dim, i - 1].bounds);
leftNum += _bins[dim, i - 1].enter;
rightNum -= _bins[dim, i - 1].exit;
float sah = nodeSAH + leftBounds.Area * leftNum + _rightBounds[i - 1].Area * rightNum;
if (sah < split.sah)
{
split.sah = sah;
split.dim = dim;
split.pos = origin[dim] + binSize[dim] * i;
}
}
}
return(split);
}