Drawing bounding rectangles around multiple objects in binary image in python

Question

I am trying to write some easy code in python to produce bounding rectangles around objects in a binary image, where there may be 1 or more objects. This is fairly easy to achieve with cv2.boundingRect for a single object, or to draw a single rectangle around 2 objects, but it does not seem to handle the multiple separate objects case. For example see the image below:

I would like to get 2 bounding boxes that define the x/y/width/height (or alternatively x1/x2/y1/y2) for EACH object separately. Does anyone know how to do this? Thanks!

Solution

The simplest way to do that in Python/OpenCV is to get the contours. Then loop over each contour and get its bounding box and draw it on the image and print it.

Input:

import cv2
import numpy as np

# read image
img = cv2.imread('two_blobs.jpg')

# convert to grayscale
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

# threshold
thresh = cv2.threshold(gray,128,255,cv2.THRESH_BINARY)[1]

# get contours
result = img.copy()
contours = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
contours = contours[0] if len(contours) == 2 else contours[1]
for cntr in contours:
    x,y,w,h = cv2.boundingRect(cntr)
    cv2.rectangle(result, (x, y), (x+w, y+h), (0, 0, 255), 2)
    print("x,y,w,h:",x,y,w,h)
 
# save resulting image
cv2.imwrite('two_blobs_result.jpg',result)      

# show thresh and result    
cv2.imshow("bounding_box", result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Bounding boxes image:

Texual results:

x,y,w,h: 262 267 37 45
x,y,w,h: 212 143 97 55