How to find contours in dotted text captcha image

I am newbie to OpenCV. I’m trying to find the contours of the captcha image. It does not work only when my captcha image contains the dotted text.

I have done following code for that:

import numpy as np
import cv2 as cv
import imgaug.augmenters as iaa

im = cv.imread('dataset/1.jpg')
imgray = cv.cvtColor(im, cv.COLOR_BGR2GRAY)

imgray = cv.threshold(imgray, 127, 255, 0)[1]

dst = cv.Canny(imgray,0,150)
blured = cv.blur(dst,(5,5),0)
img_thresh = cv.adaptiveThreshold(blured,255,cv.ADAPTIVE_THRESH_GAUSSIAN_C, cv.THRESH_BINARY_INV, 11, 2)

kernel = cv.getStructuringElement(cv.MORPH_RECT, (3,3))
threshed = cv.morphologyEx(img_thresh,cv.MORPH_CLOSE,kernel)

contours, hierarchy = cv.findContours(dst, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)
print(len(contours))
# cv.drawContours(im, contours, -1, (0, 255, 0), 3)

cv.imshow("img_thresh",img_thresh)
cv.imshow("dst",dst)
cv.imshow("threshed",threshed)
cv.waitKey(0)
cv.destroyAllWindows()

Can anyone help in this? Is there any way to find contours in this image?

Answer

Here is my code and output

'''
contours
'''
import numpy as np
import cv2

#read image as gray
pic = r'C:UsersbalajiDesktopcaptcha.jpg'
img_color = cv2.imread(pic)
cv2.imshow('CAPTCHA preview',img_color)
cv2.waitKey(0)

img_gray = cv2.cvtColor(img_color,cv2.COLOR_BGR2GRAY)

#Apply thresholding to the image
ret, thresh = cv2.threshold(img_gray, 127, 255, cv2.THRESH_OTSU)
cv2.imshow('Thresholded image', thresh)
cv2.waitKey(0)

#Dilated image - to connect the dots
krn = np.ones((3,3), np.uint8)
img_dilated = cv2.dilate(cv2.bitwise_not(thresh), kernel=krn, iterations=1)
cv2.imshow('Dilated image', img_dilated)
cv2.waitKey(0)

# Finding and draw Contours
contours, hierarchy = cv2.findContours(img_dilated, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
black_canvas = np.zeros_like(img_color)
cv2.drawContours(black_canvas, contours, -1, (0, 255, 0), 1)
cv2.imshow('Contoured image', black_canvas)
cv2.waitKey(0)