c++ - Unable to detect ArUco markers with OpenCV 3.1.0 -

i trying code simple c++ routine first write predefined dictionary of aruco markers (e.g. 4x4_100) folder , detect aruco markers in specific image selected folder using opencv 3.1 , visual studio 2017. have compiled opencv-contrib libraries required use aruco markers. routine builds without error, having trouble detecting markers after supplying correct arguments (e.g. image, dictionary, etc.) in-built "aruco::detectmarkers" function. please me understand what`s wrong approach? below minimal working example , test image attached here "4x4marker_40.jpg":

#include "opencv2\core.hpp" #include "opencv2\imgproc.hpp" #include "opencv2\imgcodecs.hpp" #include "opencv2\aruco.hpp" #include "opencv2\highgui.hpp" #include <sstream> #include <fstream> #include <iostream>  using namespace cv; using namespace std;  // function write aruco markers void createarucomarkers() {  // define variable store output markers     mat outputmarker; // choose predefined dictionary of markers     ptr< aruco::dictionary> markerdictionary = aruco::getpredefineddictionary(aruco::predefined_dictionary_name::dict_4x4_50); // write each of markers '.jpg' image file     (int = 0; < 50; i++)     {         aruco::drawmarker(markerdictionary, i, 500, outputmarker, 1);         ostringstream convert;         string imagename = "4x4marker_";         convert << imagename << << ".jpg";         imwrite(convert.str(), outputmarker);      } }   // main body of routine int main(int argv, char** argc) {     createarucomarkers();  // read specific image     mat frame = imread("4x4marker_40.jpg", cv_load_image_unchanged); // define variables store output of marker detection     vector<int> markerids;     vector<vector<point2f>> markercorners, rejectedcandidates; // define dictionary type variable marker detection       ptr<aruco::dictionary> markerdictionary = aruco::getpredefineddictionary(aruco::predefined_dictionary_name::dict_4x4_50);  // detect markers     aruco::detectmarkers(frame, markerdictionary, markercorners, markerids);  // display image     namedwindow("webcam", cv_window_autosize);     imshow("webcam", frame); // draw detected markers on displayed image     aruco::drawdetectedmarkers(frame, markercorners, markerids);     cout << "\nmarker id is:\t"<<markerids.size();     waitkey();  } 

there few problems in code:

1. you displaying image imshow before calling drawdetectedmarkers you'll never see detected marker.
2. you displaying size of markerids vector instead of value contained within it.
3. (this main problem) marker has no white space around it's impossible detect.

one suggestion: use forward slashes, not backslashes in #include statements. forward slashes work everywhere, backslashes work on windows.

this worked on machine. note loaded image color image make easier see results of drawdetectedmarkers.

#include <opencv2/core.hpp> #include <opencv2/imgproc.hpp> #include <opencv2/imgcodecs.hpp> #include <opencv2/aruco.hpp> #include <opencv2/highgui.hpp> #include <sstream> #include <fstream> #include <iostream>  using namespace cv; using namespace std;  // function write aruco markers void createarucomarkers() {      // create image hold marker plus surrounding white space     mat outputimage(700, 700, cv_8uc1);     // fill image white     outputimage = scalar(255);     // define roi write marker     rect markerrect(100, 100, 500, 500);     mat outputmarker(outputimage, markerrect);      // choose predefined dictionary of markers     ptr< aruco::dictionary> markerdictionary = aruco::getpredefineddictionary(aruco::predefined_dictionary_name::dict_4x4_50);     // write each of markers '.jpg' image file     (int = 0; < 50; i++)     {         //draw marker roi         aruco::drawmarker(markerdictionary, i, 500, outputmarker, 1);         ostringstream convert;         string imagename = "4x4marker_";         convert << imagename << << ".jpg";         // note writing outputimage, not outputmarker         imwrite(convert.str(), outputimage);      } }   // main body of routine int main(int argv, char** argc) {     createarucomarkers();      // read specific image     mat frame = imread("4x4marker_40.jpg", cv_load_image_color);     // define variables store output of marker detection     vector<int> markerids;     vector<vector<point2f>> markercorners, rejectedcandidates;     // define dictionary type variable marker detection     ptr<aruco::dictionary> markerdictionary = aruco::getpredefineddictionary(aruco::predefined_dictionary_name::dict_4x4_50);      // detect markers     aruco::detectmarkers(frame, markerdictionary, markercorners, markerids);      // display image     namedwindow("webcam", cv_window_autosize);     // draw detected markers on displayed image     aruco::drawdetectedmarkers(frame, markercorners, markerids);     // show image detected marker     imshow("webcam", frame);      // if marker identified, show id     if (markerids.size() > 0) {         cout << "\nmarker id is:\t" << markerids[0] << endl;     }     waitkey(0);  }