C++ compare surf descriptors algorithm match breaks some times

Question

I wrote a c++ application that compares points matches from images (OpenSurf C++) but some times, 1 from some thousands of "getUniqueMatches", the application breaks in some point inside "getUniqueMatches". I has this log:

05/13/11 10:17:16: this->pointsA = 227
05/13/11 10:17:16: this->pointsB = 226
05/13/11 10:17:16: this->matches before = 0
05/13/11 10:17:16: 227 226 0 0.650000
05/13/11 10:17:16: Starting in getUniqueMatches

-- And here breaks, inside getUniqueMatches --

And this is the code:

inline bool findInVector(std::vector<int> v, int value) {
    int size = v.size();
    for(int i=0; i<size; i++) {
        if(v[i] == value) {
            return true;
        }
    }

    return false;
}

void getUniqueMatches(IpVec &ipts1, IpVec &ipts2, IpPairVec &matches, float ratio) {
    try {
        wLog(f("%d %d %d %f", ipts1.size(), ipts2.size(), matches.size(), ratio));

        float dist, d1, d2;
        Ipoint *match;

        matches.clear();

        std::vector<int> matched;

        wLog("Starting in getUniqueMatches");

        // Breaks after here

        int size = ipts1.size();
        int size2 =  ipts2.size();

        for (int i = 0; i < size; i++) {
            d1 = d2 = FLT_MAX;

            int foundJ = -1;

            for (unsigned int j = 0; j < size2; j++) {
                dist = ipts1[i] - ipts2[j];

                if (dist < d1 && !findInVector(matched, j)) {
                    d2 = d1;
                    d1 = dist;
                    match = &ipts2[j];
                    foundJ = j;
                } else if (dist < d2) {
                    d2 = dist;
                }
            }

            if (d1 / d2 < ratio) {
                ipts1[i].dx = match->x - ipts1[i].x;
                ipts1[i].dy = match->y - ipts1[i].y;

                matches.push_back(std::make_pair(ipts1[i], *match));
                matched.push_back(foundJ);
            }
        }
    } catch(std::exception ex) {
        wLog(f("Exception in getUniqueMatches: ", ex.what()));
        return;
    }
}

Only some times breaks here. I don't know what is happening, maybe something wrong? The application uses only 1 thread when executing this function. When extracting points uses 10 threads.

Using it on Centos5 (VPS). 2 Gb RAm 20% hd used Compiled using g++ (Performance mode), IDE used Netbeans. OpenCV, libcurl.

Answer 1

I suspect 2 things:

• g++ compiler optimization. Use -O2 or less; -O3 and beyond sometimes generate weird behavior especially concerning floating point operation.

• If reducing optimization does not fix the problem, I suggest changing

  if (d1 / d2 < ratio)

  to

  if (d1 < double(ratio)*d2)

  for avoiding division by zero (unlikely happen I think) and for better precision result.

Answer 2

Change

if (d1 / d2 < ratio) {

for

if(d2 > 0 && ...) {

I think that the problem was a division by zero, but was not throwing exceptions. :(