feat: add mask-clear button and reject too-small / too-large watermarks

- Clear Mask button wipes paint layers while keeping the loaded frame
- mask_to_bbox: reject masks under 100 px (filters stray clicks)
- compute_crop_region: reject bboxes covering >50% of frame area
(inpainting models need surrounding context to reconstruct)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app.py

@@ -428,6 +428,17 @@ def on_preview_crop(editor_value: dict | None, meta_state: dict | None, context_
         return gr.update(), f"❌ {e}"
 
 
+def on_clear_mask(editor_value: dict | None):
+    """Clear all paint layers from the editor while preserving the loaded frame."""
+    if editor_value is None:
+        return gr.update(), "Upload a video to begin."
+    bg = editor_value.get("background")
+    return (
+        gr.update(value={"background": bg, "layers": [], "composite": None}),
+        "Mask cleared. Draw over the watermark to start again.",
+    )
+
+
 @spaces.GPU(duration=180)
 def _inpaint_composite_save_gpu(
     frame_paths: list,
@@ -635,6 +646,10 @@ with gr.Blocks(title="Video Watermark Remover", css=CSS) as demo:
 
     # ── Action buttons ───────────────────────────────────────────────────────
     with gr.Row():
+        clear_btn = gr.Button(
+            "🧹 Clear Mask",
+            elem_classes=["btn-secondary"],
+        )
         preview_btn = gr.Button(
             "🔍 Preview Crop Region",
             elem_classes=["btn-secondary"],
@@ -677,6 +692,12 @@ with gr.Blocks(title="Video Watermark Remover", css=CSS) as demo:
         outputs=[editor, crop_preview, meta_state, status_box],
     )
 
+    clear_btn.click(
+        fn=on_clear_mask,
+        inputs=[editor],
+        outputs=[editor, status_box],
+    )
+
     preview_btn.click(
         fn=on_preview_crop,
         inputs=[editor, meta_state, context_slider],

pipeline/crop.py

@@ -204,6 +204,16 @@ def mask_to_bbox(mask: np.ndarray) -> BBox:
             "The mask has no drawn pixels. "
             "Please draw around the watermark before processing."
         )
+    # Reject stray clicks / accidental dots — a real watermark covers more
+    # than ~100 pixels even when scaled down. This prevents an errant
+    # 1-pixel mask from producing a min_crop_dim crop centred on noise.
+    MIN_MASK_AREA_PX = 100
+    if len(xs) < MIN_MASK_AREA_PX:
+        raise ValueError(
+            f"The drawn area is too small ({len(xs)} px). "
+            f"Please paint over the watermark with the brush "
+            f"(minimum {MIN_MASK_AREA_PX} pixels)."
+        )
 
     return BBox(
         x1=int(xs.min()),
@@ -312,6 +322,22 @@ def compute_crop_region(
             f"Watermark bbox {watermark_bbox} is larger than the frame "
             f"({frame_w}x{frame_h}). Check your mask."
         )
+    # Reject impractically large watermarks. Inpainting models (both LaMa
+    # and VACE) are designed for localised removal; a watermark covering
+    # most of the frame leaves no scene context to reconstruct from and
+    # produces useless output.
+    bbox_area = watermark_bbox.width * watermark_bbox.height
+    frame_area = frame_w * frame_h
+    MAX_BBOX_FRACTION = 0.5
+    if bbox_area > MAX_BBOX_FRACTION * frame_area:
+        raise ValueError(
+            f"Watermark area is too large "
+            f"({bbox_area / frame_area * 100:.0f}% of frame). "
+            f"This pipeline is designed for localised watermark removal; "
+            f"the model has too little surrounding context to reconstruct "
+            f"areas larger than {int(MAX_BBOX_FRACTION * 100)}% of the frame. "
+            f"Try cropping the video first or paint a tighter mask."
+        )
 
     # ------------------------------------------------------------------
     # 1. Padded required dimensions