Opencv fitellipse draws the wrong contour

Question

I want to draw the ellipse contour around the given figure append below. I am not getting the correct result since the figure consist of two lines.

I have tried the following:-

1. Read the Image
2. Convert the BGR to HSV
3. Define the Range of color blue
4. Create the inRange Mask to capture the value of between lower and upper blue
5. Find the contour & Draw the fit ellipse.

Here is the source code-

import cv2
import numpy as np

image=cv2.imread('./source image.jpg')

hsv = cv2.cvtColor(image, cv2.COLOR_BGR2HSV)

lower_blue= np.array([75, 0, 0])
upper_blue= np.array([105, 255, 255])

mask = cv2.inRange(hsv, lower_blue, upper_blue)
res=cv2.bitwise_and(image,image,mask=mask)
_,contours,_=cv2.findContours(close,cv2.RETR_LIST,cv2.CHAIN_APPROX_SIMPLE)

ellipse = cv2.fitEllipse(max(contours,key=cv2.contourArea))
cv2.ellipse(image,ellipse,(0,255,0),2)

cv2.imshow('mask',image)
cv2.waitKey(0)
cv2.destroyAllWindows()

The figure/Image below show the Expected & Actual Output-

Expected & Actual display image

Source Image Source Image

Output Contour Array Contour file

Answer 1

I try to run your code on C++ and add erosion, dilatation and convexHull for result contour:

auto DetectEllipse = [](cv::Mat rgbImg, cv::Mat hsvImg, cv::Scalar fromColor, cv::Scalar toColor)
{
    cv::Mat threshImg;
    cv::inRange(hsvImg, fromColor, toColor, threshImg);
    cv::erode(threshImg, threshImg, cv::getStructuringElement(cv::MORPH_RECT, cv::Size(3, 3)), cv::Point(-1, -1), 2);
    cv::dilate(threshImg, threshImg, cv::getStructuringElement(cv::MORPH_RECT, cv::Size(3, 3)), cv::Point(-1, -1), 2);

    std::vector<std::vector<cv::Point> > contours;
    cv::findContours(threshImg, contours, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_NONE);

    int areaThreshold = (rgbImg.cols * rgbImg.rows) / 100;

    std::vector<cv::Point> allContours;
    allContours.reserve(10 * areaThreshold);

    for (size_t i = 0; i < contours.size(); i++)
    {
        if (contours[i].size() > 4)
        {
            auto area = cv::contourArea(contours[i]);
            if (area > areaThreshold)
            {
                allContours.insert(allContours.end(), contours[i].begin(), contours[i].end());
            }
        }
    }
    if (allContours.size() > 4)
    {
        std::vector<cv::Point> hull;
        cv::convexHull(allContours, hull, false);

        cv::ellipse(rgbImg, cv::fitEllipse(hull), cv::Scalar(255, 0, 255), 2);
    }
};

cv::Mat rgbImg = cv::imread("h8gx3.jpg", cv::IMREAD_COLOR);
cv::Mat hsvImg;
cv::cvtColor(rgbImg, hsvImg, cv::COLOR_BGR2HSV);

DetectEllipse(rgbImg, hsvImg, cv::Scalar(75, 0, 0), cv::Scalar(105, 255, 255));
DetectEllipse(rgbImg, hsvImg, cv::Scalar(10, 100, 20), cv::Scalar(25, 255, 255));

cv::imshow("rgbImg", rgbImg);
cv::waitKey(0);

Result looks correct: