app.py

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

def brightness_to_white_alpha(img, bg=None, invert_opacity=False):
    if img is None:
        return None

    # Convert image to RGB and get brightness
    img = img.convert("RGB")
    img_np = np.array(img).astype(np.float32)
    brightness = np.dot(img_np[..., :3], [0.299, 0.587, 0.114])  # shape: (H, W)

    if invert_opacity:
        brightness = 255 - brightness

    # Normalize brightness to alpha (0.0–1.0), then scale back to 0–255
    alpha = (brightness / 255.0).clip(0, 1)
    alpha = (alpha * 255).astype(np.uint8)

    # Create pure white RGB
    white = np.ones_like(img_np, dtype=np.uint8) * 255  # shape: (H, W, 3)

    # Combine white RGB and computed alpha
    white_alpha = np.dstack((white, alpha)).astype(np.uint8)  # shape: (H, W, 4)

    white_img = Image.fromarray(white_alpha, mode="RGBA")

    # If background is provided, overlay white on top
    if bg:
        bg = bg.convert("RGBA").resize(white_img.size)
        white_np = np.array(white_img).astype(np.float32) / 255.0
        bg_np = np.array(bg).astype(np.float32) / 255.0

        fg_a = white_np[..., 3:4]
        bg_a = bg_np[..., 3:4]
        out_a = fg_a + bg_a * (1 - fg_a)
        out_rgb = (white_np[..., :3] * fg_a + bg_np[..., :3] * bg_a * (1 - fg_a)) / np.maximum(out_a, 1e-6)

        result = np.dstack((out_rgb, out_a)).clip(0, 1) * 255
        return Image.fromarray(result.astype(np.uint8), mode="RGBA")

    return white_img

# Gradio UI
iface = gr.Interface(
    fn=brightness_to_white_alpha,
    inputs=[
        gr.Image(type="pil", label="Input Image"),
        gr.Image(type="pil", label="Optional Background Image"),
        gr.Checkbox(label="Invert Brightness → Alpha", value=False)
    ],
    outputs=gr.Image(type="pil", label="White with Alpha"),
    title="🎛️ Brightness to White Alpha Image",
    description=(
        "Generates a pure white image where opacity = brightness of input. "
        "Darker = more transparent, lighter = more opaque. Optional overlay on background."
    )
)

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