I'm making a script that copies an "anomaly" image and pastes it in random places from an original image. Like this:

Original Imagem

Anomaly Image:

Output Imagem Example:

But at the same time that the image with the anomaly is generated, I need to generate a mask where the area of the anomaly that I pasted is white and the rest of the image is black. Like this (I did it manually in Gimp):

Output image mask example:

How can I do this automatically at the same time as the anomaly image is generated? Below the code I'm using:

from PIL import Image
import random

anomaly = Image.open("anomaly_transp.png")  # anomaly image with transparent background
image_number = 1 # number of images
w, h = anomaly.size
offset_x, offset_y = 480-w, 512-h # offsets to avoid incorrect paste area from original image

for i in range(image_number):
    original = Image.open("Crop_120.png") # original good image
    x1 = random.randint(0,offset_x)
    y1 = random.randint(0,offset_y)
    area = (x1, y1, x1+w, y1+h)
    original.paste(anomaly, area, anomaly)
    original.save("output_"+str(i)+".png") # save output image
    original.close()