public BvhTree(BvhTree <T> nodeA, BvhTree <T> nodeB, int splittingPlane)
{
this.splittingPlane = splittingPlane;
this.nodeA = nodeA;
this.nodeB = nodeB;
this.Aabb = nodeA.Aabb + nodeB.Aabb; // we can cache this because it is not allowed to change.
this.Center = Aabb.Center;
}
public static BvhTree <T> CreateNewHierachy(List <BvhTreeItemData <T> > itemsToAdd,
int maxRecursion = int.MaxValue,
int recursionDepth = 0,
SortingAccelerator accelerator = null)
{
if (accelerator == null)
{
accelerator = new SortingAccelerator();
}
int numItems = itemsToAdd.Count;
if (numItems == 0)
{
return(null);
}
if (numItems == 1)
{
return(new BvhTree <T>(itemsToAdd));
}
int bestAxis = -1;
int bestIndexToSplitOn = -1;
var axisSorter = new AxisSorter(0);
if (recursionDepth < maxRecursion)
{
if (numItems > 5000)
{
bestAxis = accelerator.NextAxis;
bestIndexToSplitOn = numItems / 2;
}
else
{
double totalIntersectCost = 0;
int skipInterval = 1;
for (int i = 0; i < numItems; i += skipInterval)
{
var item = itemsToAdd[i];
if (item.Item is IIntersectable intersectable)
{
totalIntersectCost += intersectable.GetIntersectCost();
}
else
{
totalIntersectCost += AxisAlignedBoundingBox.GetIntersectCost();
}
}
// get the bounding box of all the items we are going to consider.
AxisAlignedBoundingBox OverallBox = itemsToAdd[0].Aabb;
for (int i = skipInterval; i < numItems; i += skipInterval)
{
OverallBox += itemsToAdd[i].Aabb;
}
double areaOfTotalBounds = OverallBox.GetSurfaceArea();
double bestCost = totalIntersectCost;
Vector3 totalDeviationOnAxis = new Vector3();
double[] surfaceArreaOfItem = new double[numItems - 1];
double[] rightBoundsAtItem = new double[numItems - 1];
for (int axis = 0; axis < 3; axis++)
{
double intersectCostOnLeft = 0;
axisSorter.WhichAxis = axis;
itemsToAdd.Sort(axisSorter);
// Get all left bounds
AxisAlignedBoundingBox currentLeftBounds = itemsToAdd[0].Aabb;
surfaceArreaOfItem[0] = currentLeftBounds.GetSurfaceArea();
for (int itemIndex = 1; itemIndex < numItems - 1; itemIndex += skipInterval)
{
currentLeftBounds += itemsToAdd[itemIndex].Aabb;
surfaceArreaOfItem[itemIndex] = currentLeftBounds.GetSurfaceArea();
totalDeviationOnAxis[axis] += Math.Abs(itemsToAdd[itemIndex].Center[axis] - itemsToAdd[itemIndex - 1].Center[axis]);
}
// Get all right bounds
if (numItems > 1)
{
AxisAlignedBoundingBox currentRightBounds = itemsToAdd[numItems - 1].Aabb;
rightBoundsAtItem[numItems - 2] = currentRightBounds.GetSurfaceArea();
for (int itemIndex = numItems - 1; itemIndex > 1; itemIndex -= skipInterval)
{
currentRightBounds += itemsToAdd[itemIndex - 1].Aabb;
rightBoundsAtItem[itemIndex - 2] = currentRightBounds.GetSurfaceArea();
}
}
// Sweep from left
for (int itemIndex = 0; itemIndex < numItems - 1; itemIndex += skipInterval)
{
double thisCost = 0;
{
// Evaluate Surface Cost Equation
double costOfTwoAABB = 2 * AxisAlignedBoundingBox.GetIntersectCost(); // the cost of the two children AABB tests
// do the left cost
if (itemsToAdd[itemIndex].Item is IIntersectable intersectable)
{
intersectCostOnLeft += intersectable.GetIntersectCost();
}
else
{
intersectCostOnLeft += AxisAlignedBoundingBox.GetIntersectCost();
}
double leftCost = (surfaceArreaOfItem[itemIndex] / areaOfTotalBounds) * intersectCostOnLeft;
// do the right cost
double intersectCostOnRight = totalIntersectCost - intersectCostOnLeft;
double rightCost = (rightBoundsAtItem[itemIndex] / areaOfTotalBounds) * intersectCostOnRight;
thisCost = costOfTwoAABB + leftCost + rightCost;
}
if (thisCost < bestCost + .000000001) // if it is less within some tiny error
{
if (thisCost > bestCost - .000000001)
{
// they are the same within the error
if (axis > 0 && bestAxis != axis) // we have changed axis since last best and we need to decide if this is better than the last axis best
{
if (totalDeviationOnAxis[axis] > totalDeviationOnAxis[axis - 1])
{
// this new axis is better and we'll switch to it. Otherwise don't switch.
bestCost = thisCost;
bestIndexToSplitOn = itemIndex;
bestAxis = axis;
}
}
}
else // this is just better
{
bestCost = thisCost;
bestIndexToSplitOn = itemIndex;
bestAxis = axis;
}
}
}
}
}
}
if (bestAxis == -1)
{
// No better partition found
return(new BvhTree <T>(itemsToAdd));
}
else
{
axisSorter.WhichAxis = bestAxis;
itemsToAdd.Sort(axisSorter);
var leftItems = new List <BvhTreeItemData <T> >(bestIndexToSplitOn + 1);
var rightItems = new List <BvhTreeItemData <T> >(numItems - bestIndexToSplitOn + 1);
for (int i = 0; i <= bestIndexToSplitOn; i++)
{
leftItems.Add(itemsToAdd[i]);
}
for (int i = bestIndexToSplitOn + 1; i < numItems; i++)
{
rightItems.Add(itemsToAdd[i]);
}
var leftGroup = CreateNewHierachy(leftItems, maxRecursion, recursionDepth + 1, accelerator);
var rightGroup = CreateNewHierachy(rightItems, maxRecursion, recursionDepth + 1, accelerator);
var newBVHNode = new BvhTree <T>(leftGroup, rightGroup, bestAxis);
return(newBVHNode);
}
}
