diff --git a/app/main.py b/app/main.py
index e0e5e6b..88cf28d 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,4 +1,4 @@
-from fastapi import FastAPI, File, HTTPException, UploadFile
+from fastapi import FastAPI, File, Form, HTTPException, UploadFile
 from fastapi.responses import HTMLResponse, StreamingResponse
 from app.config import settings
 
@@ -33,30 +33,45 @@ def index():
     <div class="grid gap-6 md:grid-cols-2">
       <section class="rounded-2xl border border-slate-800 bg-slate-900 p-4">
         <input id="file" type="file" accept="image/*" class="block w-full rounded-lg border border-slate-700 bg-slate-950 p-3" />
+        <label class="mt-4 block text-sm text-slate-400">Buffer ratio</label>
+        <input id="buffer_ratio" type="number" step="0.05" min="0" max="0.5" value="0.15" class="block w-full rounded-lg border border-slate-700 bg-slate-950 p-3" />
         <button id="go" class="mt-4 rounded-lg bg-cyan-500 px-4 py-2 font-semibold text-slate-950">Process</button>
         <pre id="meta" class="mt-4 whitespace-pre-wrap rounded-lg bg-slate-950 p-3 text-xs text-slate-300"></pre>
       </section>
       <section class="rounded-2xl border border-slate-800 bg-slate-900 p-4">
         <div class="mb-3 text-sm font-semibold text-slate-400">Result</div>
-        <img id="result" class="hidden w-full rounded-xl border border-slate-800" />
+        <div class="grid gap-4">
+          <div>
+            <div class="mb-2 text-xs uppercase tracking-wide text-slate-500">Crop</div>
+            <img id="crop" class="hidden w-full rounded-xl border border-slate-800" />
+          </div>
+          <div>
+            <div class="mb-2 text-xs uppercase tracking-wide text-slate-500">Annotated source</div>
+            <img id="annotated" class="hidden w-full rounded-xl border border-slate-800" />
+          </div>
+        </div>
       </section>
     </div>
   </main>
   <script>
     const file = document.getElementById('file');
     const go = document.getElementById('go');
-    const result = document.getElementById('result');
+    const crop = document.getElementById('crop');
+    const annotated = document.getElementById('annotated');
     const meta = document.getElementById('meta');
     go.onclick = async () => {
       if (!file.files.length) return;
       const form = new FormData();
       form.append('file', file.files[0]);
+      form.append('buffer_ratio', document.getElementById('buffer_ratio').value);
       meta.textContent = 'Working...';
       const resp = await fetch('/api/focus', { method: 'POST', body: form });
       const data = await resp.json();
       meta.textContent = JSON.stringify(data, null, 2);
-      result.src = data.crop_data_url;
-      result.classList.remove('hidden');
+      crop.src = data.crop_data_url;
+      annotated.src = data.annotated_data_url;
+      crop.classList.remove('hidden');
+      annotated.classList.remove('hidden');
     };
   </script>
 </body>
@@ -66,23 +81,23 @@ def index():
 
 
 @app.post("/api/focus")
-async def focus(file: UploadFile = File(...)):
+async def focus(file: UploadFile = File(...), buffer_ratio: float = Form(0.15)):
     from app.vision import process_image
 
     try:
         payload = await file.read()
-        return process_image(payload, file.filename or "upload")
+        return process_image(payload, file.filename or "upload", buffer_ratio=buffer_ratio)
     except ValueError as exc:
         raise HTTPException(status_code=400, detail=str(exc)) from exc
 
 
 @app.post("/api/focus/image")
-async def focus_image(file: UploadFile = File(...)):
+async def focus_image(file: UploadFile = File(...), buffer_ratio: float = Form(0.15)):
     from app.vision import process_image
 
     try:
         payload = await file.read()
-        result = process_image(payload, file.filename or "upload")
+        result = process_image(payload, file.filename or "upload", buffer_ratio=buffer_ratio)
         return StreamingResponse(result["crop_bytes_io"], media_type=result["mime_type"])
     except ValueError as exc:
         raise HTTPException(status_code=400, detail=str(exc)) from exc
diff --git a/app/vision.py b/app/vision.py
index c79f21b..0863aad 100644
--- a/app/vision.py
+++ b/app/vision.py
@@ -89,6 +89,7 @@ def detect_salient_object(image: np.ndarray) -> BBox | None:
 
 def square_bbox(bbox: BBox, image_shape: tuple[int, int, int], buffer_ratio: float = 0.15) -> BBox:
     image_h, image_w = image_shape[:2]
+    buffer_ratio = max(0.0, min(buffer_ratio, 0.5))
     side = int(round(max(bbox.w, bbox.h) * (1 + buffer_ratio * 2)))
     side = max(1, min(side, image_w, image_h))
 
@@ -126,10 +127,10 @@ def _data_url(image_bytes: bytes, mime_type: str) -> str:
     return f"data:{mime_type};base64,{base64.b64encode(image_bytes).decode('ascii')}"
 
 
-def process_image(image_bytes: bytes, filename: str) -> dict[str, Any]:
+def process_image(image_bytes: bytes, filename: str, buffer_ratio: float = 0.15) -> dict[str, Any]:
     image = decode_image(image_bytes)
     bbox, method = select_primary_bbox(image)
-    square = square_bbox(bbox, image.shape)
+    square = square_bbox(bbox, image.shape, buffer_ratio=buffer_ratio)
     crop = crop_image(image, square)
     annotated = draw_square(image, square)
 
@@ -139,6 +140,7 @@ def process_image(image_bytes: bytes, filename: str) -> dict[str, Any]:
     return {
         "filename": filename,
         "method": method,
+        "buffer_ratio": buffer_ratio,
         "detected_bbox": bbox.__dict__,
         "square_bbox": square.__dict__,
         "source_size": {"width": int(image.shape[1]), "height": int(image.shape[0])},