single-painter.py

import cv2
import numpy as np
import os
from matplotlib import pyplot as plt

fig = cv2.imread('test.png')
fig = cv2.cvtColor(fig, cv2.COLOR_BGR2RGB)

lower_mask = np.array([45, 25, 153])
upper_mask = np.array([60, 62, 205])
hsv = cv2.cvtColor(fig, cv2.COLOR_RGB2HSV)
mask = cv2.inRange(hsv, lower_mask, upper_mask)

contours, hierarchy = cv2.findContours(mask, cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
areas = []
for c in range(len(contours)):
    areas.append(cv2.contourArea(contours[c]))
max_id = areas.index(max(areas))
x, y, w, h = cv2.boundingRect(contours[max_id])
fig = cv2.drawContours(fig, contours, -1, (255, 255, 255), 4)
fig = cv2.rectangle(fig, (x, y), (x + w, y + h), (255, 0, 0), 4)

plt.title('rectangle', style='italic')
plt.imshow(fig)
plt.axis('off')
plt.savefig('single-4.pdf')
plt.show()