Draw a line using OpenCV in C++

In this article, we will discuss how to draw a line using OpenCV in C++. The idea is to use the line() function from OpenCV C++ library.

Syntax:

line(img, pt1, pt2, color, thickness, lineType, shift)

Parameters:

• img: This is the image file.
• start: Start point of the line segment. The first point out of two ends of a line segment. It is a tuple of two coordinates (x-coordinate, y-coordinate)).
• end: Endpoint of the line segment. The second point out of two ends of a line segment. It is a tuple of two coordinates (x-coordinate, y-coordinate)).
• color: Color of the line to be drawn. It is a tuple representing 3 colors (B, G, R) i.e.. (Blue, Green, Red).
• thickness: Thickness of the line drawn.
• lineType: Type of the line. There are 3 types of line:
  • LINE_4: Line was drawn using 4 connected Bresenham algorithm.
  • LINE_8: Line drawn using 8 connected Bresenham algorithm.
  • LINE_AA: It draws Antialiased lines formed by using the Gaussian filter.
• nshift: It is the Number of fractional bits in the point coordinates.

Return Value: It returns an image.

Program 1:

Below is the program shows how to draw all types of lines over a self-formed background image:

C++

// C++ program for the above approach
#include <iostream>
#include <opencv2/core/core.hpp>

// Library to include for
// drawing shapes
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc.hpp>
using namespace cv;
using namespace std;

// Driver Code
int main(int argc, char** argv)
{
    // Create a blank image of size
    // (500 x 500) with black
    // background (B, G, R) : (0, 0, 0)
    Mat image(500, 500, CV_8UC3,
              Scalar(0, 0, 0));

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

        return 0;
    }

    Point p1(0, 0), p2(100, 0);
    Point p3(200, 0), p4(500, 500);
    int thickness = 2;

    // Line drawn using 8 connected
    // Bresenham algorithm
    line(image, p1, p4, Scalar(255, 0, 0),
         thickness, LINE_8);

    // Line drawn using 4 connected
    // Bresenham algorithm
    line(image, p2, p4, Scalar(0, 255, 0),
         thickness, LINE_4);

    // Antialiased line
    line(image, p3, p4, Scalar(0, 0, 255),
         thickness, LINE_AA);

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

    return 0;
}

Output:

Program 2:

Below is the program to shows how to draw a line over a loaded image:

C++

// C++ program for the above approach
#include <iostream>
#include <opencv2/core/core.hpp>

// Library to include for
// drawing shapes
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc.hpp>
using namespace cv;
using namespace std;

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

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

    Point p1(0, 0), p2(250, 250);
    int thickness = 2;

    // Line drawn using 8 connected
    // Bresenham algorithm
    line(image, p1, p2, Scalar(255, 0, 0),
         thickness, LINE_8);

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

    return 0;
}

Output: