Rectangular bounding boxes around objects in monochrome images in python?

Question

I have a set of two monochrome images [attached] where I want to put rectangular bounding boxes for both the persons in each image. I understand that cv2.dilate may help, but most of the examples I see are focusing on detecting one rectangle containing the maximum pixel intensities, so essentially they put one big rectangle in the image. I would like to have two separate rectangles.

UPDATE: This is my attempt:

import numpy as np
import cv2

im = cv2.imread('splinet.png',0)
print im.shape
kernel = np.ones((50,50),np.uint8)
dilate = cv2.dilate(im,kernel,iterations = 10)
ret,thresh = cv2.threshold(im,127,255,0)
im3,contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
plt.imshow(im,cmap='Greys_r')
#plt.imshow(im3,cmap='Greys_r')

for i in range(0, len(contours)):
    if (i % 2 == 0):
       cnt = contours[i]
       #mask = np.zeros(im2.shape,np.uint8)
       #cv2.drawContours(mask,[cnt],0,255,-1)
       x,y,w,h = cv2.boundingRect(cnt)
       cv2.rectangle(im,(x,y),(x+w,y+h),(255,255,0),5)
       plt.imshow(im,cmap='Greys_r')
       cv2.imwrite(str(i)+'.png', im)

cv2.destroyAllWindows()

And the output is attached below: As you see, small boxes are being made and its not super clear too.

Answer 1

The real problem in your question lies in selection of the optimal threshold from the monochrome image.

In order to do that, calculate the median of the gray scale image (the second image in your post). The threshold level will be set 33% above this median value. Any value below this threshold will be binarized.

This is what I got:

Now performing morphological dilation followed by contour operations you can highlight your region of interest with a rectangle.

Note:

Never set a manual threshold as you did. Threshold can vary for different images. Hence always opt for a threshold based on the median of the image.