Adding borders to your images#


Original author	Ana Huamán
Compatibility	OpenCV >= 3.0

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.

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?
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).
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] : "lena.jpg";

    // Loads an image
    src = imread( samples::findFile( 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 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

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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):
    # First we declare the variables we are going to use
    borderType = cv.BORDER_CONSTANT
    window_name = "copyMakeBorder Demo"
    imageName = argv[0] if len(argv) > 0 else 'lena.jpg'

    # Loads an image
    src = cv.imread(cv.samples.findFile(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 lena.jpg] \n')
        return -1
    # Brief how-to for this program
    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)
    # 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
    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:])

Results#

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:

Adding borders to your images#

Goal#

Theory#

Code#

Explanation#

Declare the variables#

Load an image#

Create a window#

Initialize arguments#

Loop#

Random color#

Form a border around the image#

Display the results#

Results#