app.py

import gradio as gr
from PIL import Image
import numpy as np

def white_alpha_overlay(mask_img, bg_img, invert_opacity=False):
    if mask_img is None or bg_img is None:
        return None

    # Convert and resize
    mask = mask_img.convert("RGB")
    bg = bg_img.convert("RGBA")
    mask = mask.resize(bg.size)

    # Convert brightness to alpha
    mask_np = np.array(mask).astype(np.float32)
    brightness = np.dot(mask_np[..., :3], [0.299, 0.587, 0.114])
    if invert_opacity:
        brightness = 255 - brightness
    alpha = brightness.clip(0, 255) / 255.0  # 0-1

    # Create solid white RGBA with computed alpha
    white = np.ones_like(mask_np) / 255.0  # RGB = 1.0 (white)
    white_rgba = np.dstack((white, alpha))  # shape (H, W, 4)

    # Background as float
    bg_np = np.array(bg).astype(np.float32) / 255.0

    # Blend overlay on top of background
    alpha_fg = white_rgba[..., 3:4]
    alpha_bg = bg_np[..., 3:4]
    out_alpha = alpha_fg + alpha_bg * (1 - alpha_fg)
    out_rgb = (white_rgba[..., :3] * alpha_fg + bg_np[..., :3] * alpha_bg * (1 - alpha_fg)) / np.maximum(out_alpha, 1e-6)

    # Final image
    out_img = np.dstack((out_rgb, out_alpha)).clip(0, 1) * 255
    result = Image.fromarray(out_img.astype(np.uint8), mode="RGBA")
    return result

# Gradio UI
iface = gr.Interface(
    fn=white_alpha_overlay,
    inputs=[
        gr.Image(type="pil", label="Mask Image (Used for Brightness → Alpha)"),
        gr.Image(type="pil", label="Background Image (With Alpha OK)"),
        gr.Checkbox(label="Invert Brightness to Alpha", value=False)
    ],
    outputs=gr.Image(type="pil", label="White Overlay Composite"),
    title="🕊️ Brightness-to-Alpha White Overlay",
    description=(
        "Converts the brightness of the top image into opacity, sets the image color to white, "
        "and overlays it on a background. Works with full transparency."
    )
)

if __name__ == "__main__":
    iface.launch()