import cv2
import numpy as np
img = cv2.imread('path_to_your_image', 0)
_, blackAndWhite = cv2.threshold(img, 127, 255, cv2.THRESH_BINARY_INV)
nlabels, labels, stats, centroids = cv2.connectedComponentsWithStats(blackAndWhite, None, None, None, 8, cv2.CV_32S)
sizes = stats[1:, -1] #get CC_STAT_AREA component
img2 = np.zeros((labels.shape), np.uint8)
for i in range(0, nlabels - 1):
if sizes[i] >= 50: #filter small dotted regions
img2[labels == i + 1] = 255
res = cv2.bitwise_not(img2)
cv2.imwrite('res.png', res)