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 ObjectSplit FindObjectSplit(NodeSpec spec, float nodeSAH)
{
ObjectSplit split = ObjectSplit.New();
int refIdx = _refStack.Count - spec.numRef; // CreateLeaf以后_refStack发生了变化
for (byte dim = 0; dim < 3; dim++)
{
_refComparer.sortDim = dim;
_refStack.Sort(refIdx, spec.numRef, _refComparer);
// 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 = spec.numRef - 1; i > 0; i--)
{
rightBounds.Union(_refStack[refIdx + i].bounds);
_rightBounds[i - 1] = rightBounds; // 每一个都记录下来，后面才能比较
}
// 从左到右尝试分割，比较计算得到最佳SAH
AABB leftBounds = AABB.New();
for (int i = 1; i < spec.numRef; i++)
{
leftBounds.Union(_refStack[refIdx + i - 1].bounds);
float sah = nodeSAH + leftBounds.Area * i /*左边有i个图元*/ + _rightBounds[i - 1].Area * (spec.numRef - i);
if (sah < split.sah)
{
split.sah = sah;
split.dim = dim;
split.numLeft = i;
split.leftBounds = leftBounds;
split.rightBounds = _rightBounds[i - 1];
}
}
}
return(split);
}
private void PerformObjectSplit(ref NodeSpec left, ref NodeSpec right, NodeSpec spec, ObjectSplit split)
{
int refIdx = _refStack.Count - spec.numRef;
_refComparer.sortDim = split.dim;
_refStack.Sort(refIdx, spec.numRef, _refComparer);
left.numRef = split.numLeft;
left.bounds = split.leftBounds;
right.numRef = spec.numRef - split.numLeft;
right.bounds = split.rightBounds;
}
