Color is not responsive when Drawig a Circle function inside a loop

I would like to draw circles in a loop. I want each circle to have a different color, but when I do this, I only get one color.

I tried the code on colab 4.1.2

from numpy import random
import matplotlib.pyplot as plt
import numpy as np
import cv2

mat = np.zeros((500,500,3))
def color():
   return (  random.randint(0, 255), random.randint(0, 255), random.randint(0, 255))

for _ in range(10):
   x_coord = random.choice(500)
   y_coord =  random.choice(500)
   color_person = color()
   cv2.circle(mat, (x_coord, y_coord), 10, color=color_person,thickness=20)
   
plt.imshow(mat)

Answer

You need to create mat as uint8 type:

mat = np.zeros((500,500,3)), np.uint8)

---

matplotlib conversion is:

• NumPy arrays of type float applies pixel range [0, 1].
• NumPy arrays of type uint8 applies pixel range [0, 255].

You are using plt.imshow(mat), when type of mat is float.
All values above or equal 1.0, are at maximum intensity (equivalent to 255 in uint8).
RGB triplet at maximum intensity applies white color.

---

Complete code:

from numpy import random
import matplotlib.pyplot as plt
import numpy as np
import cv2

mat = np.zeros((500, 500, 3), np.uint8)  # Create mat with type uint8 

def color():
   return (random.randint(0, 255), random.randint(0, 255), random.randint(0, 255))

for _ in range(10):
   x_coord = random.choice(500)
   y_coord = random.choice(500)
   color_person = color()
   cv2.circle(mat, (x_coord, y_coord), 10, color=color_person, thickness=20)
   
plt.imshow(mat)
plt.show()