import cv2
import numpy as np

img = cv2.imread('portu_form.jpg', cv2.IMREAD_GRAYSCALE)
norm_img = cv2.normalize(img, None, alpha=0, beta=255, norm_type=cv2.NORM_MINMAX)
_, th = cv2.threshold(norm_img, 64, 255, cv2.THRESH_BINARY)
_, contours, _ = cv2.findContours(th, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)


good_contours = []
for contour in contours:
    perimeter = cv2.arcLength(contour, True)
    approx = cv2.approxPolyDP(contour, 0.01 * perimeter, True)
    if len(approx) == 4:
        (x, y, w, h) = cv2.boundingRect(approx)
        ar = w / float(h)
        if (ar >= 0.95) and (ar < 1.05):
            if (w > 20) and (w < 100):
                good_contours.append(approx)


output = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
cv2.drawContours(output, good_contours, -1, (0,255,0), 3)
cv2.imwrite('portu_out.png', output)