import cv2
import numpy as np

THRESH_LEVEL = 20
TARGET_SIZE = (512,512)


img = cv2.imread('lungs.png', cv2.IMREAD_GRAYSCALE)

_, im_th = cv2.threshold(img, THRESH_LEVEL, 255, cv2.THRESH_BINARY)

points = cv2.findNonZero(im_th)
x,y,w,h = cv2.boundingRect(points)

roi = img[y:y+h,x:x+w]

result = cv2.resize(roi, TARGET_SIZE)

cv2.imshow("", result)
cv2.waitKey()