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The code that I am using is:

    std::vector <PhysicsBody*> physicsChildren; //containing all objects


    std::sort(physicsChildren.begin(), physicsChildren.end(), sortByLeft);
    std::vector <PhysicsBody*> activeList;

    unsigned int one;
    unsigned int two;
    for(one = 0; one < physicsChildren.size(); ++one)
        for(two = 0; two < activeList.size(); ++two)
            if (physicsChildren[one]->m_position.x + physicsChildren[one]->m_radius > activeList[two]->m_position.x - activeList[two]->m_radius)
                CheckIntersectionBetween(physicsChildren[one], activeList[two]);
            } else {

I think something is wrong because for 800 objects 309169 calls to CheckIntersectionBetween. A bruteforce would use 640000 calls, I didn't think this was much improvement (considering only objects close in the x-axis should test).

I wrote the code from reading Jitter-Physics article about SAP:

Create a new temporary list called “activeList”.
You begin on the left of your axisList, adding the first item to the activeList. 

Now you have a look at the next item in the axisList and compare it with all items
 currently in the activeList (at the moment just one):
 - If the new item’s left is greater then the current activeList-item right, then remove 
the activeList-item from the activeList 
 - otherwise report a possible collision between the new axisList-item and the current

Add the new item itself to the activeList and continue with the next item in the axisList.

What have I done wrong?

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closed as too localized by Tetrad Feb 21 '13 at 20:33

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up vote 0 down vote accepted

I misinterpreted remove the activeList-item from the activeList to mean pop_back the item just added(I had just been added an active item, it was logical! ;) ) when, because the activeList was being looped, meant remove that specific one. Hence:

activeList.erase(activeList.begin() + two);

has replaced that line. Now 11408 checks are being done for the 800 objects (55x less than brute) and this seams much more logical to me.

share|improve this answer
I was reading the exact same article, and am struggling to implement this. It seems that you did not add the "new item" to the active list when a potential overlap is found... Did you succeed in implementing this after this question ? – JamesAMD Jun 17 '15 at 11:16
That is what activeList.push_back(physicsChildren[one]); is doing. When an overlap is found, CheckIntersectionBetween is called (as instructed by the 'otherwise report a possible collision' line) – andrew Jun 17 '15 at 15:26
About an hour ago, I took a look at the source code for Jitter Physics (Collision/CollisionSystemSAP.cs in particular, for future visitors) and got it right. Thanks for commenting me back even though this question is 2 years old :) – JamesAMD Jun 17 '15 at 15:32

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