How to group and highlight group of pixels in an image using OpenCV?

If I understand correctly, you want to highlight the differences between the input and output images in a new image. To do this, you can take a quantitative approach to determine the exact discrepancies between images using the Structural Similarity Index (SSIM) which was introduced in Image Quality Assessment: From Error Visibility to Structural Similarity. This method is already implemented in the scikit-image library for image processing. You can install scikit-image with pip install scikit-image.

The skimage.measure.compare_ssim() function returns a score and a diff image. The score represents the structural similarity index between the two input images and can fall between the range [-1,1] with values closer to one representing higher similarity. But since you're only interested in where the two images differ, the diff image is what we'll focus on. Specifically, the diff image contains the actual image differences with darker regions having more disparity. Larger areas of disparity are highlighted in black while smaller differences are in gray. Here's the diff image

If you look closely, there are gray noisy areas probably due to .jpg lossy compression. So to obtain a cleaner result, we perform morphological operations to smooth the image. We would obtain a cleaner result if the images used a lossless image compression format such as .png. After cleaning up the image, we highlight the differences in green

from skimage.measure import compare_ssim
import numpy as np
import cv2

# Load images and convert to grayscale
image1 = cv2.imread('1.jpg')
image2 = cv2.imread('2.jpg')
image1_gray = cv2.cvtColor(image1, cv2.COLOR_BGR2GRAY)
image2_gray = cv2.cvtColor(image2, cv2.COLOR_BGR2GRAY)

# Compute SSIM between two images
(score, diff) = compare_ssim(image1_gray, image2_gray, full=True)

# The diff image contains the actual image differences between the two images
# and is represented as a floating point data type in the range [0,1] 
# so we must convert the array to 8-bit unsigned integers in the range
# [0,255] before we can use it with OpenCV
diff = 255 - (diff * 255).astype("uint8")

cv2.imwrite('original_diff.png',diff)

# Perform morphological operations
kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (3,3))
opening = cv2.morphologyEx(diff, cv2.MORPH_OPEN, kernel, iterations=1)
close = cv2.morphologyEx(opening, cv2.MORPH_CLOSE, kernel, iterations=1)
diff = cv2.merge([close,close,close])

# Color difference pixels
diff[np.where((diff > [10,10,50]).all(axis=2))] = [36,255,12]

cv2.imwrite('diff.png',diff)

I thing the best way is to simply threshold you image and apply Morphological Transformations.

I have got the following results.

Threashold + Morphological:

Select the largest component:

using this code:

cv::Mat result;
cv::Mat img = cv::imread("fOTmh.jpg");

//-- gray & smooth image
cv::cvtColor(img, result, cv::COLOR_BGR2GRAY);
cv::blur(result, result, cv::Size(5,5));

//-- threashold with max value of the image and smooth again!
double min, max;
cv::minMaxLoc(result, &min, &max);
cv::threshold(result, result, 0.3*max, 255, cv::THRESH_BINARY);
cv::medianBlur(result, result, 7);

//-- apply Morphological Transformations
cv::Mat se = getStructuringElement(cv::MORPH_ELLIPSE, cv::Size(11, 11));
cv::morphologyEx(result, result, cv::MORPH_DILATE, se);
cv::morphologyEx(result, result, cv::MORPH_CLOSE, se);

//-- find the largest component
vector<vector<cv::Point> > contours;
vector<cv::Vec4i> hierarchy;
cv::findContours(result, contours, hierarchy, cv::RETR_LIST, cv::CHAIN_APPROX_NONE);
vector<cv::Point> *l = nullptr;
for(auto &&c: contours){
    if (l==nullptr || l->size()< c.size())
        l = &c;
}

//-- expand and plot Rect around the largest component
cv::Rect r = boundingRect(*l);
r.x -=10;
r.y -=10;
r.width +=20;
r.height +=20;
cv::rectangle(img, r, cv::Scalar::all(255), 3);


//-- result
cv::resize(img, img, cv::Size(), 0.25, 0.25);
cv::imshow("result", img);

Python Code :

import cv2 as cv

img = cv.imread("ELA_Final.jpg")

result = cv.cvtColor(img, cv.COLOR_BGR2GRAY);
result = cv.blur(result, (5,5));

minVal, maxVal, minLoc, maxLoc = cv.minMaxLoc(result)
ret,result = cv.threshold(result, 0.3*maxVal, 255, cv.THRESH_BINARY)
median = cv.medianBlur(result, 7)

se = cv.getStructuringElement(cv.MORPH_ELLIPSE,(11, 11));
result = cv.morphologyEx(result, cv.MORPH_DILATE, se);
result = cv.morphologyEx(result, cv.MORPH_CLOSE, se);

_,contours, hierarchy = cv.findContours(result,cv.RETR_LIST, cv.CHAIN_APPROX_NONE)

x = []

for eachCOntor in contours:
    x.append(len(eachCOntor))
m = max(x)
p = [i for i, j in enumerate(x) if j == m]

color = (255, 0, 0) 
x, y, w, h = cv.boundingRect(contours[p[0]])
x -=10
y -=10
w +=20
h +=20
cv.rectangle(img, (x,y),(x+w,y+h),color, 3)

img =  cv.resize( img,( 1500, 700), interpolation = cv.INTER_AREA)
cv.imshow("result", img)
cv.waitKey(0)