Fog: current view is transparent outside its shape so explored ground stays visible

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HJhXuLD2Z6hv3Wqeq9UL9K

backend/app/vision.py

@@ -124,7 +124,7 @@ def inside_any(pt, polys) -> bool:
 
 
 def cut(path, polys, key):
-    """The part of the image inside the polygon (everything else black), cropped to its bounding box.
+    """The part of the image inside the polygon (everything else transparent), cropped to its bounding box.
     Returns (webp bytes, (x, y, w, h))."""
     hit = _cuts.fetch(key)
     if hit:
@@ -139,7 +139,8 @@ def cut(path, polys, key):
     for poly in polys:
         if len(poly) > 2:
             draw.polygon([(x - x0, y - y0) for x, y in poly], fill=255)
-    piece = Image.composite(im.crop((x0, y0, x0 + w, y0 + h)), Image.new("RGB", (w, h), (0, 0, 0)), mask)
+    piece = im.crop((x0, y0, x0 + w, y0 + h)).convert("RGBA")
+    piece.putalpha(mask)  # transparent outside the view, so remembered (dimmed) ground underneath stays visible
     out = io.BytesIO()
     piece.save(out, "WEBP", quality=82, method=1)
     res = (out.getvalue(), (x0, y0, w, h))