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FlannBasedMatcher returning different results

asked 2014-05-01 05:36:08 -0500

cdmh gravatar image

updated 2014-05-01 06:14:32 -0500

berak gravatar image

Using the FlannBasedMatcher in OpenCV, I am getting different results calling the matcher with the same parameters. Can anyone suggest what I am doing wrong please?

The code below shows a minimal example of the problem I am having - it is simplified representative of how I use the FlannBasedMatcher - this isn't real code :)

Results output each time around the loop should be identical, but they are not.

    int const k = std::min(query_descriptors.rows,
                      std::min(train_descriptors.rows, 2));

    cv::Mat query_descriptors_original = query_descriptors.clone();
    cv::Mat train_descriptors_original = train_descriptors.clone();
    for (int loop=0; loop<2; ++loop)
    {
        cv::FlannBasedMatcher matcher;
        matcher.add(std::vector<cv::Mat>(1, train_descriptors));

        std::vector<matches_t> knnMatches;
        matcher.knnMatch(query_descriptors,  knnMatches, k);

        matches.clear();
        for (auto const &knn : knnMatches)
        {
            matches.push_back(knn[0]);
            std::cout << knn[0].queryIdx << ',' << knn[0].trainIdx << '\n';
        }
        std::cout << '\n';

        assert(cv::countNonZero(query_descriptors != query_descriptors_original) == 0);
        assert(cv::countNonZero(train_descriptors != train_descriptors_original) == 0);
    }
}

The output, although I don't think it will help(?), is

0,27
1,170
2,100
3,100
4,123
5,100
6,191
7,71
8,191
9,67
10,27
11,45
12,302
13,190
14,248
15,158
16,262
17,248
18,211
19,67
20,248
21,275

0,2
1,200
2,224
3,302
4,130
5,302
6,191
7,195
8,191
9,195
10,200
11,45
12,248
13,277
14,248
15,255
16,262
17,248
18,182
19,14
20,54
21,284
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answered 2014-05-06 08:19:20 -0500

cdmh gravatar image

Ok, so the "approximate" uses a randomized kd-tree algorithm, so the randomization is makes the result non-deterministic.

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Comments

1

If you use srand() (or whatever the initialization function is for the random number generator) you can make it deterministic.

jwatte gravatar imagejwatte ( 2014-05-06 14:57:16 -0500 )edit
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Asked: 2014-05-01 05:36:08 -0500

Seen: 240 times

Last updated: May 06 '14