Draw an ellipse using OpenCV in C++

In this article, the task is to draw an ellipse using OpenCV in C++. The ellipse() function from OpenCV C++ library will be used.

Syntax:

ellipse(image, centerCoordinates, axesLength, angle, startAngle, endAngle, color, thickness, lineType, shift)

Parameters:

• image: It is the image on which ellipse is to be drawn.
• centerCoordinates: Coordinates of the center of the ellipse. (Tuple of two coordinates (X-coordinate, Y-coordinate))
• axesLength:Tuple containing the major and minor axis of the ellipse (major axis length, minor axis length).
• angle: Ellipse rotation angle in degrees.
• startAngle: Starting angle of the elliptic arc(degrees).
• endAngle: Ending angle of the elliptic arc(degrees).
• color: It is the color of the borderline of the ellipse to be drawn. A tuple representing 3 colors (B, G, R) i.e., (Blue, Green, Red).
• thickness: It is the thickness of the ellipse borderline in px. The thickness of -1 px will fill the ellipse shape by the specified color.
• lineType: Type of the line. There are 3 types of line:
  • LINE_4: Line was drawn using 4 connected Bresenham algorithm.
  • LINE_8: Line was drawn using 8 connected Bresenham algorithm.
  • LINE_AA: It draws Anti-aliased lines formed by using a Gaussian filter.
• shift: Number of fractional bits in the point coordinates.

Return Value: It returns an image.

Program 1:

Below is the C++ program demonstrating how to draw an ellipse over a self-formed background image:

C++

// C++ program to demonstrating ellipse
// over a self-formed background image
#include <iostream>
#include <opencv2/core/core.hpp>

// Drawing shapes
#include <opencv2/imgproc.hpp>

#include <opencv2/highgui/highgui.hpp>
using namespace cv;
using namespace std;

// Driver Code
int main(int argc, char** argv)
{
    // Creating a blank image with
    // white background
    Mat image(500, 500, CV_8UC3,
              Scalar(255, 255, 255));

    // Check if the image is created
    // successfully or not
    if (!image.data) {
        std::cout << "Could not open or "
                  << "find the image\n";

        return 0;
    }

    // Drawing the ellipse
    ellipse(image, Point(256, 256),
            Size(100, 50), 0, 0,
            360, Scalar(0, 255, 255),
            -1, LINE_AA);

    // Showing image inside a window
    imshow("Output", image);
    waitKey(0);

    return 0;
}

Output:

Explanation: In the above program, an ellipse is drawn at an angle of 0 degrees i.e., horizontal ellipse.

Program 2:

Below is the C++ program illustrate rectangle over a loaded image with an ellipse around the GFG logo:

C++

// C++ program to demonstrate rectangle
// over a loaded image with an ellipse
// around the GFG logo
#include <iostream>
#include <opencv2/core/core.hpp>

// Drawing shapes
#include <opencv2/imgproc.hpp>

#include <opencv2/highgui/highgui.hpp>
using namespace cv;
using namespace std;

// Driver Code
int main(int argc, char** argv)
{
    // Reading the Image
    Mat image = imread("C:/Users/harsh/Downloads/geeks.png",
                       IMREAD_COLOR);

    // Check if the image is created
    // successfully or not
    if (!image.data) {
        std::cout << "Could not open or "
                  << "find the image\n";
        return 0;
    }

    // Drawing the ellipse
    ellipse(image, Point(115, 110),
            Size(105, 55), 0, 0,
            360, Scalar(0, 255, 255),
            1, LINE_AA);

    // Show our image inside a window
    imshow("Output", image);
    waitKey(0);

    return 0;
}

Output: