// expand performs expansion of the root node, n, towards r's center. If a new
// root node is created then it is returned and n.parent is set to the new root
// node.
func (n *Node) expand(r math.Rect3) *Node {
nb := n.bounds
if nb.Empty() {
// For starting bounds we will (squarely) encapsulate the rectangle.
rsz := r.Size()
s := rsz.X
if rsz.Y > s {
s = rsz.Y
}
if rsz.Z > s {
s = rsz.Z
}
rcenter := r.Center()
s /= 2
s *= 32
startSize := math.Vec3{s, s, s}
n.bounds = math.Rect3{
Min: rcenter.Sub(startSize),
Max: rcenter.Add(startSize),
}
return nil
}
// Expansion occurs by growing the octree such that the root node becomes
// a new node whose child is the old root node. Thus we can simply
// determine in which direction the new root should be extended (by twice
// the old root's size) by comparing the centres of the old root and the
// rectangle in question.
c := nb.Center()
rc := r.Center()
sz := nb.Size()
// Expand by becoming the opposite octant of r's closest point to nb.
var ci ChildIndex
if rc.Z > c.Z {
// Top
if rc.Y > c.Y {
// Top, Front
if rc.X > c.X {
ci = TopFrontRight
} else {
ci = TopFrontLeft
}
} else {
// Top, Back
if rc.X > c.X {
ci = TopBackRight
} else {
ci = TopBackLeft
}
}
} else {
// Bottom
if rc.Y > c.Y {
// Bottom, Front
if rc.X > c.X {
ci = BottomFrontRight
} else {
ci = BottomFrontLeft
}
} else {
// Bottom, Back
if rc.X > c.X {
ci = BottomBackRight
} else {
ci = BottomBackLeft
}
}
}
expDown := ci.Bottom()
expBack := ci.Back()
expLeft := ci.Left()
fb := n.bounds
if expDown {
fb.Min.Z -= sz.Z
} else {
fb.Max.Z += sz.Z
}
if expBack {
fb.Min.Y -= sz.Y
} else {
fb.Max.Y += sz.Y
}
if expLeft {
fb.Min.X -= sz.X
} else {
fb.Max.X += sz.X
}
newRoot := &Node{
access: n.access,
bounds: fb,
level: n.level + 1,
objects: make([][]*entry, 9),
}
newRoot.children[ci] = n
n.parent = newRoot
return newRoot
}
