app.py

import gradio as gr
import numpy as np
import imageio

def dummy(img):
    imageio.imwrite("output_image.png", img["composite"])

    alpha_channel = img["layers"][0][:, :, 3]
    mask = np.where(alpha_channel == 0, 0, 255)

    return img["background"], mask

with gr.Blocks() as demo:
  with gr.Row():
    img = gr.ImageMask(sources=["upload"], layers=False, transforms=[], format="png", label="base image", show_label=True)
    img1 = gr.Image()
    img2 = gr.Image(label="mask image", show_label=True)
  btn = gr.Button()
  btn.click(dummy, img, [img1, img2])

demo.launch(debug=True)