# Quadtree for AABB collision boxes

I have a whole bunch of entities constantly being processed and testing for collisions. I've tried to optimize this by stopping certain entity types from checking each other, dead entities not being processed and entities not checking themselves, but with the more entities I had, the slower the processing goes.

What I need is a good, clean way of implementing AABB (axis aligned bounding box) quadtree support. A way to insert an AABB of any length, width and position into the quadtree, and queering an area.

Right now I have a very basic quadtree that only takes singular points but it's buggy and can only store points. If I have a wall entity, with an AABB that extends across the whole window, then the quadtree will only store it's X and Y value in one corner of the window so collisions on the other side of the window don't occur.

Right now, my quadtree is as follows:

Creation:

//--------------Quad Tree-----------------------
quadTree = new QuadTree(screenBoundry->X() + screenBoundry->W() / 2, //X
screenBoundry->Y() + screenBoundry->H() / 2, //Y
screenBoundry->W() / 2,                      //W
screenBoundry->H() / 2,                      //H
4);                                          //Capacity

//----------------------------

QuadTree::QuadTree(double x, double y, double w, double h, int capacity)
: mo_boundry(0)
, mo_i_capacity(0)
, mo_v_points()
, mo_b_divided(false)
, mo_i_arrayCounter(0)
, northEast()
, northWest()
, southEast()
, southWest()
{
mo_boundry = new Rectangle(x, y, w, h);
mo_i_capacity = capacity;
mo_b_divided = false;
}
{
delete mo_boundry;
mo_boundry = 0;
}


Insertion:

void QuadTree::ParseEntityList(std::vector<Entity*> entityList)
{
for (int i = 0; i < static_cast<signed int>(entityList.size()); ++i) {
Insert(entityList.at(i), entityList.at(i)->GetPos());
}
}

void
QuadTree::Insert(Entity* entityToReference, Vector2* entityPoint) {

//Check if point exists in boundry
if (!mo_boundry->Contains(entityPoint)) {
return;
}

//If points are under the limit then push
if (mo_i_arrayCounter < mo_i_capacity) {
mo_v_points[entityToReference] = entityPoint; //Add to map
mo_i_arrayCounter++;
}
else {
//If not divided then divide
if (!mo_b_divided) {
SubDivide();
}

northEast->Insert(entityToReference, entityPoint);
northWest->Insert(entityToReference, entityPoint);
southEast->Insert(entityToReference, entityPoint);
southWest->Insert(entityToReference, entityPoint);
}
}


Subdivision:

void
double x = mo_boundry->X();
double y = mo_boundry->Y();
double w = mo_boundry->W() / 2;
double h = mo_boundry->H() / 2;

//Northwest boundry (x + w, y - h, w, h)
//Northeast boundry (x - w, y - h, w, h)
//Southwest boundry (x + w, y + h, w, h)
//Southeast boundry (x - w, y + h, w, h)

northEast = new QuadTree(x + w, y - h, w, h, mo_i_capacity);
northWest = new QuadTree(x - w, y - h, w, h, mo_i_capacity);
southEast = new QuadTree(x + w, y + h, w, h, mo_i_capacity);
southWest = new QuadTree(x - w, y + h, w, h, mo_i_capacity);

mo_b_divided = true;
}


Querying:

std::vector<Entity*>
QuadTree::Query(Rectangle* range, std::vector<Entity*> found)
{
//The current range doesn't overlap this boundry
if (!range->Overlaps(this->mo_boundry)) {
return found;
}

//For each point in this current quad
for (std::map<Entity*, Vector2*>::iterator it = mo_v_points.begin(); it != mo_v_points.end(); ++it) {
if (range->Contains(it->second)) {
found.push_back(it->first);
}
}

if (mo_b_divided) {
this->northEast->Query(range, found);
this->northWest->Query(range, found);
this->southEast->Query(range, found);
this->southWest->Query(range, found);
}

return found;
}


Clearing per frame:

void
{
mo_v_points.clear();

if (mo_b_divided) {
this->northEast->ClearTree();
delete northEast;
northEast = 0;

this->northWest->ClearTree();
delete northWest;
northWest = 0;

this->southEast->ClearTree();
delete southEast;
southEast = 0;

this->southWest->ClearTree();
delete southWest;
southWest = 0;
}

mo_i_arrayCounter = 0;
mo_b_divided = false;
}


My Current solutions has a bunch of issue though.

1. Memory leaks like crazy.
2. For some reason, only stores points on the right side of the screen.
3. Doesn't support, or store AABB information for collision detection.

If anyone can give some direct answers, or some links to exactly this issue, then that would be great. I've searched around for a while now, and haven't found a solution for my exact problem or the ones I have found are usually extremely unreadable or very difficult to implement into my current solution.

Thanks heaps for any answers/help I can get

• On memory leaks: there is the rule of three in C++. You class works with pointers and has a custom destructor so you should apply this rule. And, I don't see your destructor calling ClearTree or deleting anything other than rectangle. It may be the cause of leaks. – trollingchar Sep 26 '19 at 6:20
• You haven't found any references to a QuadTree implementation that stores AABBs? I have a very hard time believing that. – Tyyppi_77 Sep 26 '19 at 7:21
• Minor comment on code: using a pointer as a key in a map is legal but do keep in mind it compares the pointers address and not the logical object - might lead to unexpected behaviour. Also, prefer unordered_map over map if order is not a concern. – Mikael Sep 26 '19 at 8:01
• @trollingchar this actually massively helped. Thank you so much. You sent me down a spiral clearing out a bunch of memory leaks. – Aquaphor Sep 26 '19 at 9:28
• @Mikael I tried using unordered_map. Actually found slightly more computational heavy than a normal map for some reason. I'll have to do testing. – Aquaphor Sep 26 '19 at 9:29

There are 2 ways to deal with boxes straddling a boundary. One is to keep the box in the vector of the node where it straddles the boundary and don't push it down. The other is to add the box to all subtrees it overlaps with.

For the second option all you need to do is change first if in insert to:

//Check if box overlaps boundary
if (!mo_boundry.Overlaps(entityAABB)) {
return;
}


As an optimization when a large box gets inserted the overlaps the boundary you can try to move one of the smaller boxes in the local vector to the subtrees. Especially when the smaller box it's already fully contained within one of the subtrees. This does not need to happen during subdivision but can be done as you insert more Entities.

For the first you need to additionally check if the rectangle would overlap the boundary and if so don't push to the subtree. Here that previous optimization is mandatory.

A few more remarks:

you don't use the Mapness of your mo_v_points. Change it to a std::vector<std::pair<Entity*, Rectangle>> (You could even remove the Rectangle and get it from Entity in the loop). It's much more performant than the cache thrashing that std::map forces on you. Even if you need it as a map doing a quick linear search (std::find_if) on the vector is going to be faster than std::map::find for this use case.

Rectangle doesn't need to be held by pointer. Store it by value instead. (I suspect it's the same for the position in Entity).

I also see a lot of passing by pointer, for the pointer to Entity that is logical, for the position and range it isn't. Replace it with passing by const ref or by value for the small trivial structs.

Also you pass the result vector by value, this causes quite a few issues. For one it means that the subtrees don't actually add their results to the result vector but instead to a private copy. Instead pass the result vector by pointer. Or move the vector in to avoid the deep copy and capture the result:

found = this->northEast->Query(range, std::move(found));
found = this->northWest->Query(range, std::move(found));
found = this->southEast->Query(range, std::move(found));
found = this->southWest->Query(range, std::move(found));


To fix your leaks replace the raw pointer member fields with std::unique_ptr where you need the owning pointer semantics and by value where you don't need pointer semantics. Non-owning pointer can remain raw pointers (like the per-node vector holding pointers to Entities).

• As an optimization when a large box gets inserted the overlaps the boundary you can try to move one of the smaller boxes in the local vector to the subtrees. Especially when the smaller box it's already fully contained within one of the subtrees. This is a bit confusing. Do you mean that when a tree is subdivided, remove all items from the parent tree and split down into the lower branches? – Aquaphor Sep 26 '19 at 10:12
• Also, this is a fantastic answer. I'll definitely go through your optimization tips and try add them. I managed to get it working with your way. I was worried though that adding one entity to more than one leaf might be problematic so I added a max depth value that stops it from subdividing too far. – Aquaphor Sep 26 '19 at 10:15
• Sorry for all the comments Also you pass the result vector by value, this causes quite a few issues. For one it means that the subtrees don't actually add their results to the result vector but instead to a private copy. Instead pass the result vector by pointer. Or move the vector in to avoid the deep copy and capture the result: Yes I noticed this when I started testing. I made it into a reference pass and just created the found vector outside, above the query. Now the query directly fills up the found vector, and then I can directly use it in my physics. – Aquaphor Sep 26 '19 at 10:21
• As an addition, when you delete pointers there is no need to set them to 0 afterwards. You might want to add it to the initialization step, but then you should prefer to assign them to nullptr rather than 0 (or NULL). – Mikael Sep 26 '19 at 10:40