Returning similarity between 2 Images with KnnMatch

Question

I have the following sample from the emgu opencv wrapper

HomographyMatrix homography = null;
SURFDetector surfCPU = new SURFDetector(500,true);
        VectorOfKeyPoint modelKeyPoints;
        VectorOfKeyPoint observedKeyPoints;
        Matrix<int> indices;

        Matrix<byte> mask;
        int k = 2;
        double uniquenessThreshold = 0.9; //0.8

        //extract features from the object image
        modelKeyPoints = surfCPU.DetectKeyPointsRaw(modelImage, null);
        Matrix<float> modelDescriptors = surfCPU.ComputeDescriptorsRaw(modelImage, null, modelKeyPoints);
        modelImage.Dispose();                     


        // extract features from the observed image
        observedKeyPoints = surfCPU.DetectKeyPointsRaw(observedImage, null);
        Matrix<float> observedDescriptors = surfCPU.ComputeDescriptorsRaw(observedImage, null, observedKeyPoints);
        observedImage.Dispose();
        BruteForceMatcher<float> matcher = new BruteForceMatcher<float>(DistanceType.L2);
        matcher.Add(modelDescriptors);

        indices = new Matrix<int>(observedDescriptors.Rows, k);
        using (Matrix<float> dist = new Matrix<float>(observedDescriptors.Rows, k))
        {
            matcher.KnnMatch(observedDescriptors, indices, dist, k, null);
            mask = new Matrix<byte>(dist.Rows, 1);
            mask.SetValue(255);
            Features2DToolbox.VoteForUniqueness(dist, uniquenessThreshold, mask);
        }
        //...

after applying the KnnMatch how can i get the number of matched key points and what does the non zero pixle count has to do with getting similarity between 2 images?

Answer 1

1. refer to opencv doc http://www.opencv.org.cn/opencvdoc/2.3.2/html/modules/features2d/doc/common_interfaces_of_descriptor_matchers.html

   void DescriptorMatcher::knnMatch(const Mat& queryDescriptors, vector>& matches, int k, const vector& masks=vector(), bool compactResult=false )

   matches ( which is dist in your code) – Matches. Each matches[i] is k or less matches for the same query descriptor. k – Count of best matches found per each query descriptor or less if a query descriptor has less than k possible matches in total.

2. about similarity(I am not clear about the question), the code is using L2 distance.

   BruteForceMatcher matcher = new BruteForceMatcher(DistanceType.L2);