OpenCV flann.h assertion Error

Question

I wrote a program which gets matches between 2 pictures. And this is the code. but if I use BruteForceMatcher> (not flann) it works.

#include <stdio.h>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"

using namespace cv;

void help()
{
    printf("\nThis program demonstrates using features2d detector, descriptor extractor and simple matcher\n"
    "Using the SURF desriptor:\n"
    "\n"
    "Usage:\n matcher_simple <image1> <image2>\n");
}

int main()
{

    Mat img1 = imread("C:\\Users\\Hayk\\Desktop\\source1.bmp", CV_LOAD_IMAGE_GRAYSCALE);
    Mat img2 = imread("C:\\Users\\Hayk\\Desktop\\source2.bmp", CV_LOAD_IMAGE_GRAYSCALE);
    if(img1.empty() || img2.empty())
    {
        printf("Can't read one of the images\n");
        return -1;
    }

    // detecting keypoints
    SurfFeatureDetector detector(6000);
    vector<KeyPoint> keypoints1, keypoints2;
    detector.detect(img1, keypoints1);
    detector.detect(img2, keypoints2);

    // computing descriptors
    SurfDescriptorExtractor extractor;
    Mat descriptors1, descriptors2;
    extractor.compute(img1, keypoints1, descriptors1);
    extractor.compute(img2, keypoints2, descriptors2);

    // matching descriptors
    FlannBasedMatcher matcher;
    vector<DMatch> matches;
    matcher.match(descriptors1, descriptors2, matches);

    // drawing the results
    namedWindow("matches", 1);
    Mat img_matches;
    drawMatches(img1, keypoints1, img2, keypoints2, matches, img_matches);
    imshow("matches", img_matches);
    waitKey(0);

    return 0;
} 

And this error I am getting after running the program

OpenCV Error: Assertion failed (dataset.type() == CvType<T>::type()) in unknown
function, file c:\Users\vp\work\ocv\opencv\modules\features2d\..\flann\include\o
pencv2/flann/flann.hpp, line 105

Can anyone say to me what I am doing wrong?

Answer 1

You said it is works with the BruteForceMatcher but not Flann. I created a minimal example. Could you please run this and tell me, whether you get the same error that you got in your question?

Just trying to pin down the problem.

#include <stdio.h>
#include <iostream>
#include "opencv2/core/core.hpp"
#include "opencv2/features2d/features2d.hpp"
#include "opencv2/highgui/highgui.hpp"

using namespace cv;

int main()
{
    // Five descriptors with random values between 0 and 0.2
    Mat descriptors(5, 64, CV_32FC1);
    RNG rng;
    rng.fill(descriptors, cv::RNG::UNIFORM, 0.0, 0.2);


    // the query descriptor should yield a match in row 3 of the train descriptors
    Mat query_descriptors;
    query_descriptors.push_back(descriptors.row(3));

    // Match using Brute Force. On your machine this should work. 
    BruteForceMatcher< L2<float> > brute_matcher;
    vector<DMatch> matches;
    brute_matcher.match(query_descriptors, descriptors, matches);
    std::cout << "Brute Force Matcher: " << std::endl;
    for(int i=0; i<matches.size(); ++i)
    {
        std::cout << matches[i].queryIdx << " <--> " << matches[i].trainIdx << std::endl;
    }


    // The code should fail here because we are now going to use the FlannBasedMatcher
    std::cout << "Flann Based Matcher: " << std::endl;
    FlannBasedMatcher matcher;
    matcher.match(query_descriptors, descriptors, matches);
    for(int i=0; i<matches.size(); ++i)
    {
        std::cout << matches[i].queryIdx << " <--> " << matches[i].trainIdx << std::endl;
    }

    return 0;
} 

Answer 2

Your code works just fine on Linux with OpenCV 2.3.1a.

I've had several problems with the C++ OpenCV's interface for Windows. When I have to use Windows, I use the C interface.