# How to make a pairwise_matches array in stitching_detailed?

Recently I am trying to make some modification to the stitching_detailed cpp.

I have a group of images and I know their rough positions so I think it is possible to calculate the pairwise_matches two images by two images.

    vector<std::string> tileNames;
std::map <std::string, int> tileIndex; // index in tileNames
vector<cv::Mat> images;
vector<cv::detail::ImageFeatures> features;
vector<cv::detail::MatchesInfo> pairwise_matches;

for (int i = 0; i < tileNames.size(); i++)
{
vector<cv::detail::ImageFeatures> featuresMatching(2);
featuresMatching[0] = features[tileIndex[tileNames[i]]];

for (auto adjacentElement: tile.adjacentTiles)
{
featuresMatching[1] = features[tileIndex[ adjacentElement.name() ]];
vector<cv::detail::MatchesInfo> pairwise_matchesMatching;

(*featureMatcher)(featuresMatching, pairwise_matchesMatching);
featureMatcher->collectGarbage();

for (auto pairwiseMatchInfo : pairwise_matchesMatching)
{
if (pairwiseMatchInfo.dst_img_idx != -1 && pairwiseMatchInfo.src_img_idx !=-1)
{
pairwiseMatchInfo.dst_img_idx = featuresMatching[pairwiseMatchInfo.dst_img_idx].img_idx;
pairwiseMatchInfo.src_img_idx = featuresMatching[pairwiseMatchInfo.src_img_idx].img_idx;

pairwise_matches.push_back(pairwiseMatchInfo);
}
}
}


However, when I use the function

    cv::Ptr<cv::detail::Estimator> estimator;
estimator = cv::makePtr<cv::detail::HomographyBasedEstimator>();
vector<cv::detail::CameraParams> cameras;

(*estimator)(features, pairwise_matches, cameras);


I got a segment fault. I think the problem is that there is something wrong with the index. so I checked the matchInfo. However I am more confused.

1. MatchesInfo.dst_img_idx and MatchesInfo.src_img_idx can be -1, what does it mean?

2. In Matchesinfo.matches (a vector of Dmatch), Dmatch.imgIdx is always equal with -1 or 0. What does it mean?

3. match.imgIdx, match.queryIdx, match.trainIdx, match.distance, there is nothing in the document related with these things. I think the segment error might be caused by these since I did not update them while making a new vector, the "pairwise_matches". But what should I do to make it right?

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