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Hi guys，I am a frensh man in opencv developers，I read the book “Mastering OpenCV with Practical Computer Vision Projects”，in the three character，I find a problem，the book use ORB as detector，Freak as descriptorextractor，and the BFMatcher as matcher，when i call the knnMatch function，the program pop a bug。here is my code：

cv::Mat patternImg = cv::imread("starry_night_small.jpg", 0);//starry_night
cv::Mat testImg = cv::imread("starry_night_test.jpg", 0);
std::vector<cv::KeyPoint> queryKeypoints;
std::vector<cv::KeyPoint> trainKeypoints;
std::vector<std::vector<cv::DMatch>> matches;
cv::Mat queryDescriptor;
cv::Mat trainDescriptor;
cv::ORB detector(1000);
cv::FREAK extractor(false,false);   
cv::BFMatcher matcher(cv::NORM_HAMMING, true);
detector.detect(patternImg, trainKeypoints);
detector.detect(testImg, queryKeypoints);
extractor.compute(patternImg, trainKeypoints, trainDescriptor);
extractor.compute(testImg, queryKeypoints, queryDescriptor);

matcher.knnMatch(queryDescriptor, trainDescriptor, matches, 1);
cv::imshow("pattern", patternImg);
cout << "matches size-->" << matches.size() << endl;
cv::waitKey(0);

but when i run the program,which break and tell me a error:

OpenCV Error: Assertion failed (K == 1 && update == 0 && mask.empty()) in cv::ba tchDistance, file ......\sources\modules\core\src\stat.cpp, line 2500 ##

can you tell me how can i do

cv::Mat patternImg = cv::imread("starry_night_small.jpg", 0);//starry_night
cv::Mat testImg = cv::imread("starry_night_test.jpg", 0);
std::vector<cv::KeyPoint> queryKeypoints;
std::vector<cv::KeyPoint> trainKeypoints;
std::vector<std::vector<cv::DMatch>> matches;
cv::Mat queryDescriptor;
cv::Mat trainDescriptor;
cv::ORB detector(1000);
cv::FREAK extractor(false,false);   
cv::BFMatcher matcher(cv::NORM_HAMMING, true);
detector.detect(patternImg, trainKeypoints);
detector.detect(testImg, queryKeypoints);
extractor.compute(patternImg, trainKeypoints, trainDescriptor);
extractor.compute(testImg, queryKeypoints, queryDescriptor);

matcher.knnMatch(queryDescriptor, trainDescriptor, matches, 1);
cv::imshow("pattern", patternImg);
cout << "matches size-->" << matches.size() << endl;
cv::waitKey(0);

OpenCV Error: Assertion failed (K == 1 && update == 0 && mask.empty()) in cv::ba
tchDistance, cv::batchDistance, file ......\sources\modules\core\src\stat.cpp, ..\..\..\sources\modules\core\src\stat.cpp, line 2500
 ##

Hi guys，I guys， I am a frensh man in opencv developers，I developers， I read the book “Mastering OpenCV with Practical Computer Vision Projects”，in Projects”， in the three character，I character， I find a problem，the problem， the book use ORB ORB as detector，Freak detector， FREAK as descriptorextractor，and the BFMatcher DescriptorExtractor， and the BFMatcher as matcher，when matcher. When I call the knnMatch function，the function， the program pop a bug。here bug. Here is my code：

cv::Mat patternImg = cv::imread("starry_night_small.jpg", 0);//starry_night
cv::Mat testImg = cv::imread("starry_night_test.jpg", 0);
std::vector<cv::KeyPoint> queryKeypoints;
std::vector<cv::KeyPoint> trainKeypoints;
std::vector<std::vector<cv::DMatch>> matches;
cv::Mat queryDescriptor;
cv::Mat trainDescriptor;
cv::ORB detector(1000);
cv::FREAK extractor(false,false);   
cv::BFMatcher matcher(cv::NORM_HAMMING, true);
detector.detect(patternImg, trainKeypoints);
detector.detect(testImg, queryKeypoints);
extractor.compute(patternImg, trainKeypoints, trainDescriptor);
extractor.compute(testImg, queryKeypoints, queryDescriptor);

matcher.knnMatch(queryDescriptor, trainDescriptor, matches, 1);
cv::imshow("pattern", patternImg);
cout << "matches size-->" << matches.size() << endl;
cv::waitKey(0);

but when I run the program, which crashes and shows the error:

OpenCV Error: Assertion failed (K == 1 && update == 0 && mask.empty()) in cv::batchDistance, file ..\..\..\sources\modules\core\src\stat.cpp, line 2500

can you help me how can I fix this?