OpenCV's fitEllipse() sometimes returns completely wrong ellipses

Question

My goal is to recognize all the shapes present in an image. The idea is:

1. Extract contours
2. Fit each contour with different shapes
3. The correct

Answer 1

If you are having problems with cv::fitEllipse(), this post discuss a few methods to minimize those errors that happen when the cv::RotatedRect is draw directly without any further tests. Turns out cv::fitEllipse() is not perfect and can have issues as noted in the question.

Now, it's not entirely clear what the constraints of the project are, but another way to solve this problem is to separate these shapes based on the area of the contours:

This approach is extremely simple yet efficient on this specific case: the area of a circle varies between 1300-1699 and the area of a triangle between 1-1299.

#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include 

int main()
{
    cv::Mat img = cv::imread("input.png");
    if (img.empty())
    {
        std::cout << "!!! Failed to open image" << std::endl;
        return -1;
    }

    /* Convert to grayscale */

    cv::Mat gray;
    cv::cvtColor(img, gray, cv::COLOR_BGR2GRAY);

    /* Convert to binary */

    cv::Mat thres;
    cv::threshold(gray, thres, 127, 255, cv::THRESH_BINARY);

    /* Find contours */

    std::vector > contours;
    cv::findContours(thres, contours, cv::RETR_LIST, cv::CHAIN_APPROX_SIMPLE);

    int circles = 0;
    int triangles = 0;
    for (size_t i = 0; i < contours.size(); i++)
    {
        // Draw a contour based on the size of its area:
        //  - Area > 0 and < 1300 means it's a triangle;
        //  - Area >= 1300 and < 1700 means it's a circle;

        double area = cv::contourArea(contours[i]);
        if (area > 0 && area < 1300)
        {
            std::cout << "* Triangle #" << ++triangles << " area: " << area << std::endl;
            cv::drawContours(img, contours, i, cv::Scalar(0, 255, 0), -1, 8); // filled (green)
            cv::drawContours(img, contours, i, cv::Scalar(0, 0, 255), 2, 8); // outline (red)
        }
        else if (area >= 1300 && area < 1700)
        {
            std::cout << "* Circle #" << ++circles << " area: " << area << std::endl;
            cv::drawContours(img, contours, i, cv::Scalar(255, 0, 0), -1, 8); // filled (blue)
            cv::drawContours(img, contours, i, cv::Scalar(0, 0, 255), 2, 8); // outline (red)
        }
        else
        {
            std::cout << "* Ignoring area: " << area << std::endl;
            continue;
        }

        cv::imshow("OBJ", img);
        cv::waitKey(0);
    }   

    cv::imwrite("output.png", img);
    return 0;
}

You can invoke other functions to draw more precise outline (borders) of the shapes.