how to add border around an image in opencv python

Question

If I have an image like below, how can I add border all around the image such that the overall height and width of the final image increases but the height and width of the original image stays as-is in the middle.

Answer 1

The following code adds a constant border of size 10 pixels to all four sides of your original image.

For the colour, I have assumed that you want to use the average gray value of the background, which I have calculated from the mean value of bottom two lines of your image. Sorry, somewhat hard coded, but shows the general how-to and can be adapted to your needs.

If you leave bordersize values for bottom and right at 0, you even get a symmetric border.

Other values for BORDER_TYPE are possible, such as BORDER_DEFAULT, BORDER_REPLICATE, BORDER_WRAP.

For more details cf: http://docs.opencv.org/trunk/d3/df2/tutorial_py_basic_ops.html#gsc.tab=0

import numpy as np
import cv2

im = cv2.imread('image.jpg')
row, col = im.shape[:2]
bottom = im[row-2:row, 0:col]
mean = cv2.mean(bottom)[0]

bordersize = 10
border = cv2.copyMakeBorder(
    im,
    top=bordersize,
    bottom=bordersize,
    left=bordersize,
    right=bordersize,
    borderType=cv2.BORDER_CONSTANT,
    value=[mean, mean, mean]
)

cv2.imshow('image', im)
cv2.imshow('bottom', bottom)
cv2.imshow('border', border)
cv2.waitKey(0)
cv2.destroyAllWindows()