# How can I optimize a collision engine where order is significant and collision is conditional based on object group?

If this is your first time on this question, I suggest reading the pre-update part below first, then this part. Here's a synthesis of the problem, though:

Basically, I have a collision detection and resolution engine with a grid spatial partitioning system where order-of-collision and collision groups matter. One body at a time must move, then detect collision, then resolve collisions. If I move all the bodies at once, then generate possible collision pairs, it is obviously faster, but resolution breaks because order-of-collision isn't respected. If I move one body a time, I'm forced to get bodies to check collisions, and it becomes a n^2 problem. Put groups in the mix, and you can imagine why it gets very slow very fast with a lot of bodies.

Update: I've worked really hard on this, but couldn't manage to optimize anything.

I successfully implemented the "painting" described by Will and changed groups into bitsets, but it is a very very minor speedup.

I also discovered a big issue: my engine is order-of-collision dependent.

I tried an implementation of unique collision pair generation, which definitely speed up everything by a lot, but broke the order-of-collision.

Let me explain:

• in my original design (not generating pairs), this happens:

1. a single body moves
2. after it has moved, it refreshes its cells and gets the bodies it collides against
3. if it overlaps a body it needs to resolve against, resolve the collision

this means that if a body moves, and hits a wall (or any other body), only the body that has moved will resolve its collision and the other body will be unaffected.

This is the behavior I desire.

I understand it's not common for physics engines, but it has a lot of advantages for retro-style games.

• in the usual grid design (generating unique pairs), this happens:

1. all bodies move
2. after all bodies have moved, refresh all cells
3. generate unique collision pairs
4. for each pair, handle collision detection and resolution

in this case a simultaneous move could have resulted in two bodies overlapping, and they will resolve at the same time - this effectively makes the bodies "push one another around", and breaks collision stability with multiple bodies

This behavior is common for physics engines, but it is not acceptable in my case.

I also found another issue, which is major (even if it's not likely to happen in a real-world situation):

• consider bodies of group A, B and W
• A collides and resolves against W and A
• B collides and resolves against W and B
• A does nothing against B
• B does nothing against A

there can be a situation where a lot of A bodies and B bodies occupy the same cell - in that case, there is a lot of unnecessary iteration between bodies that mustn't react to one another (or only detect collision but not resolve them).

For 100 bodies occupying the same cell, it's 100^100 iterations! This happens because unique pairs aren't being generated - but I can't generate unique pairs, otherwise I would get a behavior I do not desire.

Is there a way to optimize this kind of collision engine?

These are the guidelines that must be respected:

• Order of collision is extremely important!

• Bodies must move one at a time, then check for collisions one at a time, and resolve after movement one at a time.
• Bodies must have 3 group bitsets

• Groups: groups the body belongs to
• GroupsToCheck: groups the body must detect collision against
• GroupsNoResolve: groups the body must not resolve collision against
• There can be situations where I only want a collision to be detected but not resolved

Pre-update:

Foreword: I'm aware that optimizing this bottleneck is not a necessity - the engine is already very fast. I, however, for fun and educational purposes, would love to find a way to make the engine even faster.

I'm creating a general-purpose C++ 2D collision detection/response engine, with an emphasis on flexibility and speed.

Here's a very basic diagram of its architecture:

Basically, the main class is World, which owns (manages memory) of a ResolverBase*, a SpatialBase* and a vector<Body*>.

SpatialBase is a pure virtual class which deals with broad-phase collision detection.

ResolverBase is a pure virtual class which deals with collision resolution.

The bodies communicate to the World::SpatialBase* with SpatialInfo objects, owned by the bodies themselves.

There currenly is one spatial class: Grid : SpatialBase, which is a basic fixed 2D grid. It has it's own info class, GridInfo : SpatialInfo.

Here's how its architecture looks:

The Grid class owns a 2D array of Cell*. The Cell class contains a collection of (not owned) Body*: a vector<Body*> which contains all the bodies that are in the cell.

GridInfo objects also contain non-owning pointers to the cells the body is in.

As I previously said, the engine is based on groups.

• Body::getGroups() returns a std::bitset of all the groups the body is part of.
• Body::getGroupsToCheck() returns a std::bitset of all the groups the body has to check collision against.

Bodies can occupy more than a single cell. GridInfo always stores non-owning pointers to the occupied cells.

After a single body moves, collision detection happens. I assume that all bodies are axis-aligned bounding boxes.

Part 1: spatial info update

For each Body body:

• Top-leftmost occupied cell and bottom-rightmost occupied cells are calculated.
• If they differ from the previous cells, body.gridInfo.cells is cleared, and filled with all the cells the body occupies (2D for loop from the top-leftmost cell to the bottom-rightmost cell).
1. body is now guaranteed to know what cells it occupies.

Part 2: actual collision checks

For each Body body:

• body.gridInfo.handleCollisions is called:

void GridInfo::handleCollisions(float mFrameTime)
{
static int paint{-1};
++paint;

for(const auto& c : cells)
for(const auto& b : c->getBodies())
{
if(b->paint == paint) continue;
base.handleCollision(mFrameTime, b);
b->paint = paint;
}
}


void Body::handleCollision(float mFrameTime, Body* mBody)
{
if(mBody == this || !mustCheck(*mBody) || !shape.isOverlapping(mBody->getShape())) return;

auto intersection(getMinIntersection(shape, mBody->getShape()));

onDetection({*mBody, mFrameTime, mBody->getUserData(), intersection});
mBody->onDetection({*this, mFrameTime, userData, -intersection});

if(!resolve || mustIgnoreResolution(*mBody)) return;
bodiesToResolve.push_back(mBody);
}


• Collision is then resolved for every body in bodiesToResolve.

• That's it.

So, I've been trying to optimize this broad-phase collision detection for quite a while now. Every time I try something else than the current architecture/setup, something doesn't go as planned or I make assumption about the simulation that later are proven to be false.

My question is: how can I optimize the broad-phase of my collision engine?

Is there some kind of magic C++ optimization that can be applied here?

Can the architecture be redesigned in order to allow for more performance?

• Profile and identify bottlenecks. Let us know where they are, then we have something to work on. – Maik Semder Jun 24 '13 at 20:05
• @MaikSemder: I did that, and wrote it in the post. It's the only code snippet that's the bottleneck. Sorry if it's long and detailed, but that's part of the question because I'm sure that this bottleneck can only be solved by changing something in the design of the engine. – Vittorio Romeo Jun 24 '13 at 20:09
• Sorry, was hard to find. Can you give us some numbers? The function time and number of objects processed in that function? – Maik Semder Jun 24 '13 at 20:15
• @MaikSemder: tested with Callgrind, on a binary compiled with Clang 3.4 SVN -O3: 10000 dynamic bodies - the function getBodiesToCheck() was called 5462334 times, and took 35,1% of the entire profiling time (Instruction read access time) – Vittorio Romeo Jun 24 '13 at 20:30
• @Quonux: no offense taken. I just love "reinventing the wheel". I could take Bullet or Box2D and make a game with those libraries, but that's not really my goal. I feel much more fulfilled and learn much more by creating things from scratch and trying to overcome the obstacles that appear - even if that means being frustrated and asking for help. Other than my belief that coding from scratch is invaluable for learning purposes, I also find it a lot of fun, and a great pleasure to spend my free time on. – Vittorio Romeo Jul 7 '13 at 1:05

getBodiesToCheck()

There could be two problems with the getBodiesToCheck() function; first:

if(!contains(bodiesToCheck, b)) bodiesToCheck.push_back(b);


This part is O(n2) isn't it?

Rather than checking to see if the body is already in the list, use painting instead.

loop_count++;
if(!loop_count) { // if loop_count can wrap,
// you just need to iterate all bodies to reset it here
}
bodiesToCheck.clear();
for(const auto& q : queries)
for(const auto& b : *q)
if(b->paint != loop_count) {
bodiesToCheck.push_back(b);
b->paint = loop_count;
}
return bodiesToCheck;


You are dereferencing the pointer in the gather phase, but you'd be dereferencing it in the test phase anyway so if you have enough L1 its no big deal. You can improve performance by adding pre-fetch hints to the compiler too e.g. __builtin_prefetch, although that is easier with classic for(int i=q->length; i-->0; ) loops and such.

That's a simple tweak, but my second thought is that there could be a faster way to organise this:

You can move to using bitmaps instead, though, and avoiding the whole bodiesToCheck vector. Here's an approach:

You are using integer keys for bodies already, but then looking them up in maps and things and keeping around lists of them. You can move to a slot allocator, which is basically just an array or vector. E.g.:

class TBodyImpl {
public:
virtual ~TBodyImpl() {}
virtual void onHit(int other) {}
virtual ....
const int slot;
protected:
TBodyImpl(int slot): slot(slot_) {}
};

struct TBodyBase {
enum ... type;
...
rect_t rect;
TBodyImpl* imp; // often null
};

std::vector<TBodyBase> bodies; // not pointers to them


What this means is that all the stuff needed to do the actual collisions is in linear cache-friendly memory, and you only go out to the implementation-specific bit and attach it to one of these slots if there's some need to.

To track the allocations in this vector of bodies you can use an array of integers as a bitmap and use bit twiddling or __builtin_ffs etc. This is super efficient to move to slots that are currently occupied, or find an unoccupied slot in the array. You can even compact the array sometimes if it grows unreasonably large and then lots are marked deleted, by moving those on the end to fill in the gaps.

only check for each collision once

If you've checked if a collides with b, you don't need to check if b collides with a too.

It follows from using integer ids that you avoid these checks with a simple if-statement. If the id of a potential collision is less-than-or-equal to the current id being checked for, it can be skipped! This way, you'll only ever check each possible pairing once; that'll more than half the number of collision checks.

unsigned * bitmap;
int bitmap_len;
...

for(int i=0; i<bitmap_len; i++) {
const int slot = i*sizeof(unsigned)*8+j;
for(int neighbour: get_neighbours(slot))
if(neighbour > slot)
check_for_collision(slot,neighbour);
}


respect the order of collisions

Rather than evaluating a collision as soon as a pair is found, compute the distance to hit and store that in a binary heap. These heaps are how you typically do priority queues in path-finding, so is very useful utility code.

Mark each node with a sequence number, so you can say:

• A10 hits B12 at 6
• A10 hits C12 at 3

Obviously after you've gathered all the collisions, you start popping them from the priority queue, soonest first. So the first you get is A10 hits C12 at 3. You increment each object's sequence number (the 10 bit), evaluate the collision, and compute their new paths, and store their new collisions in the same queue. The new collision is A11 hits B12 at 7. The queue now has:

• A10 hits B12 at 6
• A11 hits B12 at 7

Then you pop from the priority queue and its A10 hits B12 at 6. But you see that A10 is stale; A is currently at 11. So you can discard this collision.

Its important not to bother trying to delete all stale collisions from the tree; removing from a heap is expensive. Simply discard them when you pop them.

the grid

You should consider using a quadtree instead. Its a very straightforward data-structure to implement. Often you see implementations that store points but I prefer to store rects, and store the element in the node that contains it. This means that to check collisions you only have to iterate over all bodies, and, for each, check it against those bodies in the same quad-tree node (using the sorting trick outlined above) and all those in parent quad-tree nodes. The quad-tree is itself the possible-collision list.

struct Object {
Rect bounds;
Point pos;
Object * prev, * next;
};

Rect bounds;
Point centre;
Object * staticObjects;
Object * movableObjects;
QuadTreeNode * parent; // null if root
QuadTreeNode * children[4]; // null for unallocated children
};


We store the movable objects separately because we don't have to check if the static objects are going to collide with anything.

We are modeling all objects as axis-aligned bounding boxes (AABB) and we put them in the smallest QuadTreeNode that contains them. When a QuadTreeNode a lot of children, you can subdivide it further (if those objects distribute themselves into the children nicely).

Each game tick, you need to recurse into the quadtree and compute the move - and collisions - of each movable object. It has to be checked for collisions with:

• every static object in its node
• every movable object in its node that is before it (or after it; just pick a direction) in the movableObjects list
• every movable and static object in all parent nodes

This will generate all possible collisions, unordered. You then do the moves. You have to prioritise these moves by distance and 'who moves first' (which is your special requirement), and execute them in that order. Use a heap for this.

You can optimise this quadtree template; you don't need to actually store the bounds and centre-point; that's entirely derivable when you walk the tree. You don't need to check if a model is within the bounds, only check which side it is of the centre-point (a "axis of separation" test).

To model fast flying things like projectiles, rather than moving them each step or having a separate 'bullets' list that you always check, simply put them in the quadtree with the rect of their flight for some number of game steps. This means that they move in the quadtree much more rarely, but you aren't checking bullets against far off walls, so its a good tradeoff.

Large static objects should be split into component parts; a large cube should have each face separately stored, for example.

• "Painting" sounds good, I'll give it a try and report results as soon as possible. I don't understand the second part of your answer though - I'll try to read something about pre-fetching. – Vittorio Romeo Jun 25 '13 at 12:47
• I wouldn't recommend QuadTree, it's more complicated than doing a grid, and if not done properly it will not work accurately and will create/remove nodes too often. – ClickerMonkey Jun 27 '13 at 11:00
• About your heap: is the order of movement respected? Consider body A and body B. A moves to the right towards B, and and B moves to the right towards A. Now - when they collide simultaneously, the one that moved first should get resolved first, and the other one would be unaffected. – Vittorio Romeo Jul 7 '13 at 11:51
• @VittorioRomeo if A moves towards B and B moves towards A in the same tick, and at the same speed, do they meet in the middle? Or does A, moving first, meet B where B starts? – Will Jul 8 '13 at 5:41
• – Vittorio Romeo Jul 10 '13 at 9:46

I bet you just have a ton of cache misses when iterating over the bodies. Are you pooling all your bodies together using some data oriented design scheme? With an N^2 broadphase I can simulate hundreds and hundreds, while recording with fraps, of bodies without any framerate drops into the nether regions (less than 60), and this is all without a custom allocator. Just imagine what can be done with proper cache usage.

The clue is here:

const std::vector<Body *>


This immediately raises a huge red flag. Are you allocating these bodies with raw new calls? Is there a custom allocator in use? It's most important that you have all your bodies in a huge array in which you traverse linearly. If traversing the memory linearly isn't something you feel you can implement consider using an intrusively linked list instead.

Additionally you seem to be using std::map. Do you know how the memory within std::map is allocated? You'll have an O( lg( N ) ) complexity for each map query, and this can likely be increased to O( 1 ) with a hash table. On top of this the memory allocated by std::map is going to thrash your cache horribly as well.

My solution is to use an intrusive hash table in place of std::map. A good example of both intrusively linked lists and intrusive hash tables is in Patrick Wyatt's base within his coho project: https://github.com/webcoyote/coho

So in short you'll probably need to create some custom tools for yourself, namely an allocator and some intrusive containers. This is the best I can do without profiling the code for myself.

• "Are you allocating these bodies with raw new calls?" I'm not explicitly calling new when pushing bodies to the getBodiesToCheck vector - do you mean it is happening internally? Is there a way to prevent that while still having a dynamic-sized collection of bodies? – Vittorio Romeo Jun 25 '13 at 9:02
• std::map is not a bottleneck - I also remember trying dense_hash_set and not gaining any kind of performance. – Vittorio Romeo Jun 25 '13 at 9:03
• @Vittorio, then which part of getBodiesToCheck is the bottleneck? We need information in order to help. – Maik Semder Jun 25 '13 at 11:16
• @MaikSemder: the profiler doesn't go deeper than the function itself. The whole function is the bottleneck, because it's being called once per frame per body. 10000 bodies = 10000 getBodiesToCheck calls per frame. I suspect the constant cleaning/pushing in the vector is the bottleneck of the function itself. The contains method is also part of the slowdown, but since bodiesToCheck never has more than 8-10 bodies in it, it should be that slow – Vittorio Romeo Jun 25 '13 at 11:36
• @Vittorio would be nice if you put this info into the questions, thats a game-changer ;) Particularily I mean the part that getBodiesToCheck is called for all bodies, so 10000 times each frame. I wonder, you said they were in groups, so why put them into the bodiesToCheck-array, if you already have the group information. You might elaborate on that part, looks like a very good optimization candidate to me. – Maik Semder Jun 25 '13 at 12:23

Reduce the the count of bodies to check each frame:

Only check bodies that can actually move. Static objects only need to be assigned to your collision cells once after being created. Now only check collisions for groups which do contain at least one dynamic object. This should reduce the number of checks each frame.

Remove all allocations from your physics code. You may wanna use a profiler for this. But I have only analysed memory allocation in C#, so I can't help with C++.

Good luck!

I see two problem candidates in your bottleneck function:

First is "contains" part - this is probably the main reason of bottleneck. It's iterating through already found bodies for every body. Maybe you should rather use some kind of hash_table / hash_map instead of vector. Then inserting should be faster (with searching of duplicities). But I don't know any specific numbers - I have no idea how many bodies are iterated here.

Second problem could be vector::clear and push_back. Clear may or may not evoke reallocation. But you may want to avoid it. Solution could be some flags array. But you may probably have a lot of objects, so it's memory ineffective to have list of all objects for every object. Some other approach could be nice, but I don't know what approach :/

• About the first problem: I've tried using a dense_hash_set instead of the vector+contains, and it was slower. I tried filling the vector and then removing all duplicates, and it was slower. – Vittorio Romeo Jun 25 '13 at 8:58

Note: I know nothing of C++, only Java, but you should be able to figure out the code. Physics is universal language right? I also realize this is a year old post, yet I just wanted to share this with everyone.

I have an observer pattern that basically, after the entity moves, it returns the object it has collided with, including a NULL object. Simply put:

(I am remaking minecraft)

public Block collided(){
return World.getBlock(getLocation());
}


So say you are wandering around in your world. whenever you call move(1) you then call collided(). if you get the block you want, then perhaps particles fly and you can move left right and back but not forward.

Using this more generically than just minecraft as an example:

public Object collided(){
return threeDarray[3][2][3];
}


Simply, have an array to point out the coordinates which, literally how Java does it, uses pointers.

Using this method still requires something other than a priori method of collision detection. You could loop this, but that defeats the purpose. You can apply this to Broad-, mid-, and narrow-collision techniques, but alone, its a beast especially when it works for 3D and 2D games quite well.

Now taking one more look, this means that, according to my minecraft collide() method, I will end up inside the block, so I will have to move the player outside of it. Instead of checking the player, I need to add a bounding box which checks which block is hitting each side of the box. Problem fixed.

the above paragraph may not be so easy with polygons if you want accuracy. For accuracy I would suggest defining a polygon bounding box that isn't a square, but not tesselated. if not, then a rectangle is just fine.