How to draw a rectangle on the Aruco marker in opencv?

I have a program to draw and detect the aruco markers, and write the marker id on it. I need a rectangle to appear on each marker instead of the marker id, i could draw a rectangle but in a fixed position not on the marker, here is the code:

#include <opencv2\highgui.hpp>
#include <opencv2\aruco.hpp>
#include <opencv2\core.hpp>
#include <opencv2\imgcodecs.hpp>
#include <opencv2\imgproc.hpp>
#include <opencv2\calib3d.hpp>
#include <sstream>
#include <fstream>
#include <iostream>
using namespace cv;
using namespace std;


int main(int argc, char *argv[]) {
    cv::VideoCapture inputVideo;
    inputVideo.open(0);
    Mat outputMarker;
    auto  markerDict = aruco::getPredefinedDictionary(aruco::PREDEFINED_DICTIONARY_NAME::DICT_4X4_50);
    for (int i = 0; i < 50; i++) {
        aruco::drawMarker(markerDict, i, 500, outputMarker, 1);
        ostringstream convert;
        String imageName = "4x4marker_";
        convert << imageName << i << ".jpg";
        imwrite(convert.str(), outputMarker);

        while (inputVideo.grab()) {
            cv::Mat image, imageCopy;
            inputVideo.retrieve(image);
            image.copyTo(imageCopy);

            std::vector<int> ids;
            std::vector<std::vector<cv::Point2f> > corners;
            cv::aruco::detectMarkers(image, markerDict, corners, ids);

            // if at least one marker detected
            if (ids.size() > 0)
                cv::aruco::drawDetectedMarkers(imageCopy, corners, ids);
            int x = 0;
            int y = 3;


            rectangle(imageCopy, Point(imageCopy.cols/2, imageCopy.rows/2),
                Point(x,y),Scalar::all(255), -1, 8, 0);



            cv::imshow("out", imageCopy);
            char key = (char)cv::waitKey(5);
            if (key == 27)
                break;
        }
    }
}

An example marker for easing code testing.

Solution

When you use the detectMarkers function, it returns the corners of each detection. In your case, you are putting it into the std::vector<std::vector<cv::Point2f> > corners. To draw the rectangles you ask for, you can do something similar to (the following code is an example and has not been tested):

for (size_t i = 0; i< corners.size(); +i)
{
  cv::Point2f p0(image.cols,image.rows);
  cv::Ponit2f p1(0,0);
  for (auto p: corners[i])
  {
    if (p.x < p0.x)
      p0.x = p.x;
    if (p.y < p0.y)
      p0.y = p.y;
    if (p.x > p1.x)
      p1.x = p.x;
    if (p.y > p1.y)
      p1.y = p.y;
  }
  rectangle(imageCopy, p0, p1,Scalar::all(255), -1, 8, 0);
}

However, you probably want to draw a polygon that will better fit the marker, as the marker projection on the image will not be a rectangle if the marker is not perfectly perpendicular to the camera optical axis and there's no distortion. For that, you can use fillPoly, or if you don't want it filled line.

Here is a fully functional example tested with OpenCV 4.3.0-pre and 3.2.0 in Ubuntu and 3.4.9 in Windows:

#include <iostream>

#include <opencv2/aruco.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>

int main(int argc, char** argv)
{
  // Check cv versionmake
  std::cout << "Using OpenCV version: " << CV_VERSION << std::endl;

  // Create video input
  cv::VideoCapture inputVideo;
  int input_source(0);
  if (argc > 1)
    input_source = std::atoi(argv[1]);
  if (!inputVideo.open(input_source))
  {
    std::cerr << "Error opening input video soruce: " << input_source << std::endl;
    return EXIT_FAILURE;
  }

  // Create marker dictionary
  auto marker_dict = cv::aruco::getPredefinedDictionary(cv::aruco::PREDEFINED_DICTIONARY_NAME::DICT_4X4_50);

  // Get imshow ready
  cv::namedWindow("Display window", cv::WINDOW_KEEPRATIO | cv::WINDOW_NORMAL);
  cv::resizeWindow("Display window", 800, 600);

  // Grab images until escape is pressed
  int key = 0;
  while (key != 27 && inputVideo.grab())
  {
    // Retrieve image
    cv::Mat image;
    inputVideo.retrieve(image);

    // Get image output ready
    cv::Size image_size = image.size();
    cv::Mat out_image(image_size.height, 3 * image_size.width, CV_8UC3);
    cv::Mat left(out_image, cv::Rect(0, 0, image_size.width, image_size.height));
    image.copyTo(left);
    cv::Mat mid(out_image, cv::Rect(image_size.width, 0, image_size.width, image_size.height));
    image.copyTo(mid);
    cv::Mat right(out_image, cv::Rect(2 * image_size.width, 0, image_size.width, image_size.height));
    image.copyTo(right);

    // Add names to images
    int corner_offset = 50;
    cv::putText(left, "Original image", cv::Point(corner_offset, corner_offset), cv::FONT_HERSHEY_DUPLEX, 1.0,
                CV_RGB(0, 0, 0), 2);
    cv::putText(mid, "Image with OpenCV drawing", cv::Point(corner_offset, corner_offset), cv::FONT_HERSHEY_DUPLEX, 1.0,
                CV_RGB(0, 0, 0), 2);
    cv::putText(right, "Image with custom drawing", cv::Point(corner_offset, corner_offset), cv::FONT_HERSHEY_DUPLEX,
                1.0, CV_RGB(0, 0, 0), 2);

    // Detect markers
    std::vector<int> ids;
    std::vector<std::vector<cv::Point2f> > corners;
    cv::aruco::detectMarkers(image, marker_dict, corners, ids);

    // Draw markers using opencv tool
    cv::aruco::drawDetectedMarkers(mid, corners, ids);

    // Draw markers custom
    for (size_t i = 0; i < corners.size(); ++i)
    {
      // Convert to integer ponits
      int num = static_cast<int>(corners[i].size());
      std::vector<cv::Point> points;
      for (size_t j = 0; j < corners[i].size(); ++j)
        points.push_back(cv::Point(static_cast<int>(corners[i][j].x), static_cast<int>(corners[i][j].y)));
      const cv::Point* pts = &(points[0]);

      // Draw
      cv::fillPoly(right, &pts, &num, 1, cv::Scalar(255, 0, 0));

      // Draw contour
      for (size_t j = 0; j < corners[i].size(); ++j)
      {
        size_t next = (j + 1) % corners[i].size();
        cv::line(right, corners[i][j], corners[i][next], cv::Scalar(0, 255, 0), 5);
      }
    }

    // Display
    cv::imshow("Display window", out_image);
    key = cv::waitKey(5);
  }

  return EXIT_SUCCESS;
}

This is the output you will get with this code: