--- a
+++ b/src/auxiliary.h
@@ -0,0 +1,48 @@
+#include "Rcpp.h"
+#include <opencv2/opencv.hpp>
+#include "opencv2/xfeatures2d.hpp"
+#include "opencv2/features2d.hpp"
+#include "opencv2/shape/shape_transformer.hpp"
+
+#ifndef AUXILIARY_H
+#define AUXILIARY_H
+
+////
+// replace NAs in matrices
+////
+
+Rcpp::NumericMatrix replaceNaMatrix(Rcpp::NumericMatrix mat, int replace);
+
+////
+// Conversion
+////
+
+// cv::Mat vs Rcpp::RawVector(Image)
+Rcpp::RawVector matToImage(cv::Mat mat);
+cv::Mat imageToMat(Rcpp::RawVector image_data, int width, int height);
+
+// cv::Mat vs Rcpp::NumericMatrix 
+cv::Mat numericMatrixToMat(Rcpp::NumericMatrix nm);
+Rcpp::NumericMatrix matToNumericMatrix(cv::Mat m);
+
+// std::vector<cv::Point2f> vs Rcpp::NumericMatrix
+std::vector<cv::Point2f> numericMatrixToPoint2f(Rcpp::NumericMatrix mat);
+Rcpp::NumericMatrix point2fToNumericMatrix(std::vector<cv::Point2f> points);
+  
+// std::vector<cv::Point2f> vs cv::Mat
+std::vector<cv::Point2f> matToPoint2f(cv::Mat mat);
+cv::Mat point2fToMat(std::vector<cv::Point2f> points);
+
+// std::vector<double> vs std::vector<cv::Point2f>
+std::vector<double> KeyPointToDoubleVector(std::vector<cv::KeyPoint> points);
+std::vector<double> Point2fToDoubleVector(std::vector<cv::Point2f> points);
+  
+////
+// validation
+////
+  
+// standard deviation
+double cppSD(std::vector<cv::KeyPoint> points);
+double cppSD(std::vector<cv::Point2f> points);
+  
+#endif
\ No newline at end of file