I tried to detect all the rectangles in image using threshold, canny edge and applied contour detection but it was not able to detect all the rectangles. Finally, I thought of detect the same using hough transforms but when I tried to detect lines in image, I'm getting all the lines. I need to detect only rectangular boxes in the image. Can someone help me? I'm new to opencv.

Input image

Code:

import cv2
import matplotlib.pyplot as plt
import numpy as npimg =  cv2.imread("demo-hand-written.png",-1)
#img = cv2.resize(img,(1280,720))
edges = cv2.Canny(img,180,200)
kernel = np.ones((2,2),np.uint8)
d = cv2.dilate(edges,kernel,iterations = 2)
e = cv2.erode(img,kernel,iterations = 2)  
#ret, th = cv2.threshold(img, 220, 255, cv2.THRESH_BINARY_INV)lines = cv2.HoughLinesP(edges,1,np.pi/180,30, maxLineGap=20,minLineLength=30)
for line in lines:#print(line)x1,y1,x2,y2 = line[0]cv2.line(img,(x1,y1),(x2,y2),(0,255,0),3)
cv2.imshow("image",img)
cv2.waitKey(0)
cv2.destroyAllWindows()

You can use below code as a starting point.

img =  cv2.imread('demo-hand-written.png')
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)thresh_inv = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)[1]# Blur the image
blur = cv2.GaussianBlur(thresh_inv,(1,1),0)thresh = cv2.threshold(blur, 0, 255, cv2.THRESH_BINARY+cv2.THRESH_OTSU)[1]# find contours
contours = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[0]mask = np.ones(img.shape[:2], dtype="uint8") * 255
for c in contours:# get the bounding rectx, y, w, h = cv2.boundingRect(c)if w*h>1000:cv2.rectangle(mask, (x, y), (x+w, y+h), (0, 0, 255), -1)res_final = cv2.bitwise_and(img, img, mask=cv2.bitwise_not(mask))cv2.imshow("boxes", mask)
cv2.imshow("final image", res_final)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output:

Figure 1: Detected rectangular boxes in the above image

Figure 2: Detected rectangular contours in the original image