Adding borders to your images

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Goal

In this tutorial you will learn how to:

• Use the OpenCV function copyMakeBorder() to set the borders (extra padding to your image).

Theory

Note

The explanation below belongs to the book Learning OpenCV by Bradski and Kaehler.

1. In our previous tutorial we learned to use convolution to operate on images. One problem that naturally arises is how to handle the boundaries. How can we convolve them if the evaluated points are at the edge of the image?
2. What most of OpenCV functions do is to copy a given image onto another slightly larger image and then automatically pads the boundary (by any of the methods explained in the sample code just below). This way, the convolution can be performed over the needed pixels without problems (the extra padding is cut after the operation is done).

3. In this tutorial, we will briefly explore two ways of defining the extra padding (border) for an image:

   1. BORDER_CONSTANT: Pad the image with a constant value (i.e. black or \(0\)
   2. BORDER_REPLICATE: The row or column at the very edge of the original is replicated to the extra border.

   This will be seen more clearly in the Code section.

• What does this program do?
  • Load an image

  • Let the user choose what kind of padding use in the input image. There are two options:

    1. Constant value border: Applies a padding of a constant value for the whole border. This value will be updated randomly each 0.5 seconds.
    2. Replicated border: The border will be replicated from the pixel values at the edges of the original image.

    The user chooses either option by pressing 'c' (constant) or 'r' (replicate)

  • The program finishes when the user presses 'ESC'

Code

The tutorial code's is shown lines below.

C++

You can also download it from here

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

using namespace cv;

// Declare the variables
Mat src, dst;
int top, bottom, left, right;
int borderType = BORDER_CONSTANT;
const char* window_name = "copyMakeBorder Demo";
RNG rng(12345);

int main( int argc, char** argv )
{
    const char* imageName = argc >=2 ? argv[1] : "../data/lena.jpg";

    // Loads an image
    src = imread( imageName, IMREAD_COLOR ); // Load an image

    // Check if image is loaded fine
    if( src.empty()) {
        printf(" Error opening image\n");
        printf(" Program Arguments: [image_name -- default ../data/lena.jpg] \n");
        return -1;
    }

    // Brief how-to for this program
    printf( "\n \t copyMakeBorder Demo: \n" );
    printf( "\t -------------------- \n" );
    printf( " ** Press 'c' to set the border to a random constant value \n");
    printf( " ** Press 'r' to set the border to be replicated \n");
    printf( " ** Press 'ESC' to exit the program \n");

    namedWindow( window_name, WINDOW_AUTOSIZE );

    // Initialize arguments for the filter
    top = (int) (0.05*src.rows); bottom = top;
    left = (int) (0.05*src.cols); right = left;

    for(;;)
    {
        Scalar value( rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255) );

        copyMakeBorder( src, dst, top, bottom, left, right, borderType, value );

        imshow( window_name, dst );

        char c = (char)waitKey(500);
        if( c == 27 )
        { break; }
        else if( c == 'c' )
        { borderType = BORDER_CONSTANT; }
        else if( c == 'r' )
        { borderType = BORDER_REPLICATE; }
    }

    return 0;
}

Java

You can also download it from here

import org.opencv.core.*;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;

import java.util.Random;

class CopyMakeBorderRun {

    public void run(String[] args) {

        // Declare the variables
        Mat src, dst = new Mat();
        int top, bottom, left, right;
        int borderType = Core.BORDER_CONSTANT;
        String window_name = "copyMakeBorder Demo";
        Random rng;

        String imageName = ((args.length > 0) ? args[0] : "../data/lena.jpg");

        // Load an image
        src = Imgcodecs.imread(imageName, Imgcodecs.IMREAD_COLOR);

        // Check if image is loaded fine
        if( src.empty() ) {
            System.out.println("Error opening image!");
            System.out.println("Program Arguments: [image_name -- default ../data/lena.jpg] \n");
            System.exit(-1);
        }

        // Brief how-to for this program
        System.out.println("\n" +
                "\t copyMakeBorder Demo: \n" +
                "\t -------------------- \n" +
                " ** Press 'c' to set the border to a random constant value \n" +
                " ** Press 'r' to set the border to be replicated \n" +
                " ** Press 'ESC' to exit the program \n");

        HighGui.namedWindow( window_name, HighGui.WINDOW_AUTOSIZE );

        // Initialize arguments for the filter
        top = (int) (0.05*src.rows()); bottom = top;
        left = (int) (0.05*src.cols()); right = left;

        while( true ) {
            rng = new Random();
            Scalar value = new Scalar( rng.nextInt(256),
                    rng.nextInt(256), rng.nextInt(256) );

            Core.copyMakeBorder( src, dst, top, bottom, left, right, borderType, value);
            HighGui.imshow( window_name, dst );

            char c = (char) HighGui.waitKey(500);
            c = Character.toLowerCase(c);

            if( c == 27 )
            { break; }
            else if( c == 'c' )
            { borderType = Core.BORDER_CONSTANT;}
            else if( c == 'r' )
            { borderType = Core.BORDER_REPLICATE;}
        }

        System.exit(0);
    }
}

public class CopyMakeBorder {
    public static void main(String[] args) {
        // Load the native library.
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
        new CopyMakeBorderRun().run(args);
    }
}

Python

You can also download it from here

"""
@file copy_make_border.py
@brief Sample code that shows the functionality of copyMakeBorder
"""
import sys
from random import randint
import cv2 as cv


def main(argv):
    
    borderType = cv.BORDER_CONSTANT
    window_name = "copyMakeBorder Demo"
    
    imageName = argv[0] if len(argv) > 0 else "../data/lena.jpg"

    # Loads an image
    src = cv.imread(imageName, cv.IMREAD_COLOR)

    # Check if image is loaded fine
    if src is None:
        print ('Error opening image!')
        print ('Usage: copy_make_border.py [image_name -- default ../data/lena.jpg] \n')
        return -1
    
    print ('\n'
           '\t   copyMakeBorder Demo: \n'
           '     -------------------- \n'
           ' ** Press \'c\' to set the border to a random constant value \n'
           ' ** Press \'r\' to set the border to be replicated \n'
           ' ** Press \'ESC\' to exit the program ')
    
    cv.namedWindow(window_name, cv.WINDOW_AUTOSIZE)
    
    top = int(0.05 * src.shape[0])  # shape[0] = rows
    bottom = top
    left = int(0.05 * src.shape[1])  # shape[1] = cols
    right = left
    
    while 1:
        
        value = [randint(0, 255), randint(0, 255), randint(0, 255)]
        
        dst = cv.copyMakeBorder(src, top, bottom, left, right, borderType, None, value)
        
        cv.imshow(window_name, dst)
        
        c = cv.waitKey(500)

        if c == 27:
            break
        elif c == 99: # 99 = ord('c')
            borderType = cv.BORDER_CONSTANT
        elif c == 114: # 114 = ord('r')
            borderType = cv.BORDER_REPLICATE
        
    return 0


if __name__ == "__main__":
    main(sys.argv[1:])

Explanation

Declare the variables

First we declare the variables we are going to use:

C++

// Declare the variables
Mat src, dst;
int top, bottom, left, right;
int borderType = BORDER_CONSTANT;
const char* window_name = "copyMakeBorder Demo";
RNG rng(12345);

Java

        // Declare the variables
        Mat src, dst = new Mat();
        int top, bottom, left, right;
        int borderType = Core.BORDER_CONSTANT;
        String window_name = "copyMakeBorder Demo";
        Random rng;

Python

    # First we declare the variables we are going to use
    borderType = cv.BORDER_CONSTANT
    window_name = "copyMakeBorder Demo"

Especial attention deserves the variable rng which is a random number generator. We use it to generate the random border color, as we will see soon.

Load an image

As usual we load our source image src:

C++

    const char* imageName = argc >=2 ? argv[1] : "../data/lena.jpg";

    // Loads an image
    src = imread( imageName, IMREAD_COLOR ); // Load an image

    // Check if image is loaded fine
    if( src.empty()) {
        printf(" Error opening image\n");
        printf(" Program Arguments: [image_name -- default ../data/lena.jpg] \n");
        return -1;
    }

Java

        String imageName = ((args.length > 0) ? args[0] : "../data/lena.jpg");

        // Load an image
        src = Imgcodecs.imread(imageName, Imgcodecs.IMREAD_COLOR);

        // Check if image is loaded fine
        if( src.empty() ) {
            System.out.println("Error opening image!");
            System.out.println("Program Arguments: [image_name -- default ../data/lena.jpg] \n");
            System.exit(-1);
        }

Python

    imageName = argv[0] if len(argv) > 0 else "../data/lena.jpg"

    # Loads an image
    src = cv.imread(imageName, cv.IMREAD_COLOR)

    # Check if image is loaded fine
    if src is None:
        print ('Error opening image!')
        print ('Usage: copy_make_border.py [image_name -- default ../data/lena.jpg] \n')
        return -1

Create a window

After giving a short intro of how to use the program, we create a window:

C++

    namedWindow( window_name, WINDOW_AUTOSIZE );

Java

        HighGui.namedWindow( window_name, HighGui.WINDOW_AUTOSIZE );

Python

    cv.namedWindow(window_name, cv.WINDOW_AUTOSIZE)

Initialize arguments

Now we initialize the argument that defines the size of the borders (top, bottom, left and right). We give them a value of 5% the size of src.

C++

    // Initialize arguments for the filter
    top = (int) (0.05*src.rows); bottom = top;
    left = (int) (0.05*src.cols); right = left;

Java

        // Initialize arguments for the filter
        top = (int) (0.05*src.rows()); bottom = top;
        left = (int) (0.05*src.cols()); right = left;

Python

    # Initialize arguments for the filter
    top = int(0.05 * src.shape[0])  # shape[0] = rows
    bottom = top
    left = int(0.05 * src.shape[1])  # shape[1] = cols
    right = left

Loop

The program runs in an infinite loop while the key ESC isn't pressed. If the user presses 'c' or 'r', the borderType variable takes the value of BORDER_CONSTANT or BORDER_REPLICATE respectively:

C++

        char c = (char)waitKey(500);
        if( c == 27 )
        { break; }
        else if( c == 'c' )
        { borderType = BORDER_CONSTANT; }
        else if( c == 'r' )
        { borderType = BORDER_REPLICATE; }

Java

            char c = (char) HighGui.waitKey(500);
            c = Character.toLowerCase(c);

            if( c == 27 )
            { break; }
            else if( c == 'c' )
            { borderType = Core.BORDER_CONSTANT;}
            else if( c == 'r' )
            { borderType = Core.BORDER_REPLICATE;}

Python

        c = cv.waitKey(500)

        if c == 27:
            break
        elif c == 99: # 99 = ord('c')
            borderType = cv.BORDER_CONSTANT
        elif c == 114: # 114 = ord('r')
            borderType = cv.BORDER_REPLICATE

Random color

In each iteration (after 0.5 seconds), the random border color (value) is updated...

C++

        Scalar value( rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255) );

Java

            rng = new Random();
            Scalar value = new Scalar( rng.nextInt(256),
                    rng.nextInt(256), rng.nextInt(256) );

Python

        value = [randint(0, 255), randint(0, 255), randint(0, 255)]

This value is a set of three numbers picked randomly in the range \([0,255]\).

Form a border around the image

Finally, we call the function copyMakeBorder() to apply the respective padding:

C++

        copyMakeBorder( src, dst, top, bottom, left, right, borderType, value );

Java

            Core.copyMakeBorder( src, dst, top, bottom, left, right, borderType, value);

Python

        dst = cv.copyMakeBorder(src, top, bottom, left, right, borderType, None, value)

• The arguments are:
  1. src: Source image
  2. dst: Destination image
  3. top, bottom, left, right: Length in pixels of the borders at each side of the image. We define them as being 5% of the original size of the image.
  4. borderType: Define what type of border is applied. It can be constant or replicate for this example.
  5. value: If borderType is BORDER_CONSTANT, this is the value used to fill the border pixels.

Display the results

We display our output image in the image created previously

C++

        imshow( window_name, dst );

Java

            HighGui.imshow( window_name, dst );

Python

        cv.imshow(window_name, dst)

Results

1. After compiling the code above, you can execute it giving as argument the path of an image. The result should be:

   • By default, it begins with the border set to BORDER_CONSTANT. Hence, a succession of random colored borders will be shown.
   • If you press 'r', the border will become a replica of the edge pixels.
   • If you press 'c', the random colored borders will appear again
   • If you press 'ESC' the program will exit.

   Below some screenshot showing how the border changes color and how the BORDER_REPLICATE option looks: