feat(api): robust painted mask detection; fallback to dark-threshold when diff empty; add logs

api/main.py

@@ -11,6 +11,10 @@ from fastapi.responses import FileResponse, JSONResponse
 from pydantic import BaseModel
 from PIL import Image
 import cv2
+import logging
+
+logging.basicConfig(level=logging.INFO)
+log = logging.getLogger("api")
 
 from src.core import process_inpaint
 
@@ -198,6 +202,15 @@ def inpaint_multipart(
             _, binmask = cv2.threshold(gray, 0, 255, cv2.THRESH_BINARY + cv2.THRESH_OTSU)
         except Exception:
             _, binmask = cv2.threshold(gray, 40, 255, cv2.THRESH_BINARY)
+        nonzero = int((binmask > 0).sum())
+        log.info("painted-mask via diff, pixels: %d", nonzero)
+        # If nothing detected (user uploaded plain original as mask), detect dark strokes directly
+        if nonzero < 50:
+            gray_painted = cv2.cvtColor(m_rgb, cv2.COLOR_RGB2GRAY)
+            # pick very dark paint
+            _, binmask = cv2.threshold(gray_painted, 60, 255, cv2.THRESH_BINARY_INV)
+            nonzero = int((binmask > 0).sum())
+            log.info("painted-mask via dark-thresh, pixels: %d", nonzero)
         # Build RGBA mask where selected area has alpha=0
         mask_rgba = np.zeros((binmask.shape[0], binmask.shape[1], 4), dtype=np.uint8)
         mask_rgba[:, :, 3] = np.where(binmask > 0, 0, 255).astype(np.uint8)