[SOLVED] Find new blobs comparing two different binary images

Instead of using OR,AND,XOR, we may sum the two images.
Before summing the images, replace the 255 values with 100 (keeping the range of uint8 [0, 255]).
In the summed image, there are going to be three values:

0 - Background
100 - Non-overlapping area
200 - Overlapping area

We may assume that pixels with value 100 that touches value 200 belongs to the same original blob.

For clearing the overlapping pixels (200) with the touching pixels (100 around them), we may use cv2.floodFill.

After clearing the overlapping pixels and the pixels around them, the pixels that are left (with value 100) are the new blobs.

Example for clearing the pixels using cv2.floodFill:

if sum_img[y, x] == 200:
    cv2.floodFill(sum_img, None, (x, y), 0, loDiff=100, upDiff=0)

Setting loDiff=100 is used for filling pixels=100 (and pixels=200) with 0 value (200-loDiff=100, so the 100 is filled with zero).

For making the solution better, we may find contours (of pixels=200), and ignore the tiny contours.

Code sample:

import cv2
import numpy as np

# Read input images as Grayscale.
img1 = cv2.imread('image1.png', cv2.IMREAD_GRAYSCALE)
img2 = cv2.imread('image2.png', cv2.IMREAD_GRAYSCALE)

# Replace 255 with 100 (we want the sum img1+img2 not to overflow)
img1[img1 >= 128] = 100
img2[img2 >= 128] = 100

# Sum two images - in the sum, the value of overlapping parts of blobs is going to be 200
sum_img = img1 + img2

cv2.floodFill(sum_img, None, (0, 0), 0, loDiff=0, upDiff=0)  # Remove the white frame.

cv2.imshow('sum_img before floodFill', sum_img)  # Show image for testing.

# Find pixels with value 200 (the overlapping blobs).
thesh = cv2.threshold(sum_img, 199, 255, cv2.THRESH_BINARY)[1]

# Find contours (of overlapping blobs parts)
cnts = cv2.findContours(thesh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[0]


# Iterate contours and fill the overlapping part, and the non-zero pixels around it (i.e with value 100) with zero.
for c in cnts:
    area_tresh = 50
    area = cv2.contourArea(c)
    if area > area_tresh:  # Ignore very tiny contours
        x, y = tuple(c[0, 0])  # Get coordinates of first pixel in the contour
        if sum_img[y, x] == 200:
            cv2.floodFill(sum_img, None, (x, y), 0, loDiff=100, upDiff=0)  # Setting loDiff=100 is set for filling pixels=100 (and pixels=200)

sum_img[sum_img == 200] = 0  # Remove the small remainders

#thesh = cv2.cvtColor(thesh, cv2.COLOR_GRAY2BGR)  # Convert to BGR for testing (for drawing the contours)
#cv2.drawContours(thesh, cnts, -1, (0, 255, 0), 2)  # Draw contours for testing

# Show images for testing.
cv2.imshow('thesh', thesh)
cv2.imshow('sum_img after floodFill', sum_img)
cv2.waitKey()
cv2.destroyAllWindows()

Note:
We may dilate the images first, if the two blobs in proximity are considered to be the same blob (I don't no if a blob can "swim")

Output sum_img (after floodFill):

Update:

The above solution finds the blobs that exist in image1 and not in image2 and blobs exist in image2 and not in image1.

In case we want to find only the blobs that are new in image2, and we also assume that blobs that are close in both images are the same one, we may add the following stages:

Dilate img1 and img2 before summing (two close blobs are going to be overlapping).
Remove the dilated pixels from sum_img at the end.
Remove from sum_img all the blobs that exist only in img1 (and not in img2).

Code sample:

import cv2
import numpy as np

# Read input images as Grayscale.
img1 = cv2.imread('image1.png', cv2.IMREAD_GRAYSCALE)
img2 = cv2.imread('image2.png', cv2.IMREAD_GRAYSCALE)

# Replace 255 with 100 (we want the sum img1+img2 not to overflow)
img1[img1 >= 128] = 100
img2[img2 >= 128] = 100

# Dilate both images - assume close blobs are the same blob (two blobs are considered overlapped even if they are close but not tuching).
dilated_img1 = cv2.dilate(img1, np.ones((11, 11), np.uint8))
dilated_img2 = cv2.dilate(img2, np.ones((11, 11), np.uint8))

# Sum two images - in the sum, the value of overlapping parts of blobs is going to be 200
sum_img = dilated_img1 + dilated_img2

cv2.floodFill(sum_img, None, (0, 0), 0, loDiff=0, upDiff=0)  # Remove the white frame.

#cv2.imshow('sum_img before floodFill', sum_img)  # Show image for testing.

# Find pixels with value 200 (the overlapping blobs).
thesh = cv2.threshold(sum_img, 199, 255, cv2.THRESH_BINARY)[1]

# Find contours (of overlapping blobs parts)
cnts = cv2.findContours(thesh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)[0]


# Iterate contours and fill the overlapping part, and the non-zero pixels around it (i.e with value 100) with zero.
for c in cnts:
    area_tresh = 0  # Optional
    area = cv2.contourArea(c)
    if area > area_tresh:  # Ignore very tiny contours
        x, y = tuple(c[0, 0])  # Get coordinates of first pixel in the contour
        if sum_img[y, x] == 200:
            cv2.floodFill(sum_img, None, (x, y), 0, loDiff=100, upDiff=0)  # Setting loDiff=100 is set for filling pixels=100 (and pixels=200)

sum_img[sum_img == 200] = 0  # Remove the small remainders

sum_img[(img1 == 0) & (dilated_img1 == 100)] = 0  # Remove dilated pixels from dilated_img1
sum_img[(img2 == 0) & (dilated_img2 == 100)] = 0  # Remove dilated pixels from dilated_img2
sum_img[(img1 == 100) & (img2 == 0)] = 0  # Remove all the blobs that are only in first image (assume new blobs are "bored" only in image2)

# Visualization:
merged_img = cv2.merge((sum_img*2, img1*2, img2*2))

# The output image is img1, without the 
output_image = img1.copy()
output_image[sum_img == 100] = 0

# Show images for testing.
cv2.imshow('sum_img', sum_img)
cv2.imshow('merged_img', merged_img)
cv2.waitKey()
cv2.destroyAllWindows()

Output (sum_img*2):

Visualization for testing (merged_img):

Green - exist only in img1
Yellow - exist both in img1 and img2
Magenta - exist only in img2, and not too close to blob in img1 (we are looking for the magenta blobs).