Update app.py

app.py

@@ -10,39 +10,54 @@ def brightness_to_opacity_overlay(foreground_img, background_img, invert_opacity
     bg = background_img.convert("RGBA")
     fg = foreground_img.convert("RGB").resize(bg.size)
 
-    # Convert foreground to grayscale (luminance)
+    # Convert to luminance (grayscale)
     fg_array = np.array(fg)
     luminance = np.dot(fg_array[...,:3], [0.299, 0.587, 0.114]).astype(np.uint8)
 
-    # Optionally invert the opacity
+    # Invert if requested
     alpha = 255 - luminance if invert_opacity else luminance
 
-    # Set RGB to white, combine with alpha
+    # Create white RGBA image with computed alpha
     white_rgb = np.ones_like(fg_array) * 255
     rgba_array = np.dstack((white_rgb, alpha)).astype(np.uint8)
-    mask_image = Image.fromarray(rgba_array, mode="RGBA")
+    overlay = Image.fromarray(rgba_array, mode="RGBA")
 
-    # Overlay mask_image on background
-    combined = Image.alpha_composite(bg, mask_image)
+    # Manual alpha blending
+    bg_arr = np.array(bg).astype(np.float32) / 255.0
+    ov_arr = np.array(overlay).astype(np.float32) / 255.0
 
-    return combined
+    # Extract alpha channels
+    alpha_bg = bg_arr[..., 3:4]
+    alpha_ov = ov_arr[..., 3:4]
+
+    # Composite alpha and color
+    out_alpha = alpha_ov + alpha_bg * (1 - alpha_ov)
+    out_rgb = (
+        ov_arr[..., :3] * alpha_ov +
+        bg_arr[..., :3] * alpha_bg * (1 - alpha_ov)
+    ) / np.clip(out_alpha, 1e-6, 1)
+
+    # Combine and convert to final image
+    out_image = np.dstack((out_rgb, out_alpha)).clip(0, 1) * 255
+    out_image = Image.fromarray(out_image.astype(np.uint8), mode="RGBA")
+
+    return out_image
 
 # Gradio UI
 iface = gr.Interface(
     fn=brightness_to_opacity_overlay,
     inputs=[
         gr.Image(type="pil", label="Mask Source (Brightness to Alpha)"),
-        gr.Image(type="pil", label="Background Image"),
+        gr.Image(type="pil", label="Background Image (with or without transparency)"),
         gr.Checkbox(label="Invert Opacity", value=False)
     ],
     outputs=gr.Image(type="pil", label="Final Composite"),
-    title="Brightness-to-Alpha Overlay Tool",
+    title="Brightness-to-Alpha Overlay Tool (with Transparency Fix)",
     description=(
-        "This tool uses the brightness of the first image as its opacity mask, "
-        "optionally inverts it, resizes it to match the second image, and overlays the result."
+        "Uses the brightness of the first image as alpha, resizes to match the second image, "
+        "and overlays correctly even when the background has transparency."
     )
 )
 
-# Run the app
 if __name__ == "__main__":
     iface.launch()