diff --git a/imgdiet/core.py b/imgdiet/core.py
index 16acce2..2e4c68a 100644
--- a/imgdiet/core.py
+++ b/imgdiet/core.py
@@ -50,65 +50,42 @@ def calculate_psnr(
     return 20.0 * math.log10(max_pixel / math.sqrt(mse))
 
 
-def measure_webp_quality_pil(
+def measure_webp_pil(
     original_bgr: np.ndarray,
     pil_image: Image.Image,
-    quality: int
+    *,
+    quality: Optional[int] = None,
+    lossless: bool = False
 ) -> Tuple[float, int, bytes]:
     """
-    Compresses the given PIL Image to WebP (quality-based), 
-    returns (psnr, compressed_size, compressed_data).
-    """
-    buffer = io.BytesIO()
-    icc_profile = pil_image.info.get("icc_profile")
+    Compresses the given PIL Image to WebP.
+    - If `lossless` is True, performs lossless compression.
+    - Otherwise, uses quality-based compression with the provided `quality`.
     
-    # Save with original mode (RGB or RGBA)
-    pil_image.save(
-        buffer,
-        format="WEBP",
-        quality=quality,
-        icc_profile=icc_profile,
-        exact=True
-    )
-    data = buffer.getvalue()
-    size = len(data)
-
-    buffer.seek(0)
-    # Open and convert to RGB only for PSNR calculation
-    compressed_pil = Image.open(buffer)
-    if compressed_pil.mode == 'RGBA':
-        compressed_pil = compressed_pil.convert('RGB')
-    compressed_bgr = np.array(compressed_pil)[:, :, ::-1]
-
-    psnr_val = calculate_psnr(original_bgr, 
-                              compressed_bgr)
-    return psnr_val, size, data
-
-
-def measure_webp_lossless_pil(
-    original_bgr: np.ndarray,
-    pil_image: Image.Image
-) -> Tuple[float, int, bytes]:
-    """
-    Compresses the given PIL Image in lossless WebP, 
-    returns (psnr, compressed_size, compressed_data).
+    Returns a tuple (psnr, compressed_size, compressed_data).
     """
+    if not lossless and quality is None:
+        raise ValueError("Either 'lossless' must be True or 'quality' must be provided for quality-based compression.")
+    
     buffer = io.BytesIO()
     icc_profile = pil_image.info.get("icc_profile")
     
-    # Save with original mode (RGB or RGBA)
-    pil_image.save(
-        buffer,
-        format="WEBP",
-        lossless=True,
-        icc_profile=icc_profile,
-        exact=True
-    )
+    # Set up parameters for saving
+    save_kwargs = {
+        "format": "WEBP",
+        "icc_profile": icc_profile,
+        "exact": True,
+    }
+    if lossless:
+        save_kwargs["lossless"] = True
+    else:
+        save_kwargs["quality"] = quality
+
+    pil_image.save(buffer, **save_kwargs)
     data = buffer.getvalue()
     size = len(data)
 
     buffer.seek(0)
-    # Open and convert to RGB only for PSNR calculation
     compressed_pil = Image.open(buffer)
     if compressed_pil.mode == 'RGBA':
         compressed_pil = compressed_pil.convert('RGB')
@@ -134,7 +111,7 @@ def find_optimal_compression_binary_search(
 
     while left <= right:
         mid = (left + right) // 2
-        psnr_val, size, _ = measure_webp_quality_pil(original_bgr, pil_image, mid)
+        psnr_val, size, _ = measure_webp_pil(original_bgr, pil_image, quality=mid, lossless=False)
 
         if psnr_val >= target_psnr:
             if size < best_size:
@@ -246,7 +223,7 @@ def process_single_image(
         # Case 1: target_psnr == 0 => lossless
         if target_psnr == 0:
             try:
-                psnr_val, compressed_size, data = measure_webp_lossless_pil(original_bgr, pil_image)
+                psnr_val, compressed_size, data = measure_webp_pil(original_bgr, pil_image, lossless=True)
                 if psnr_val == float("inf") and compressed_size < original_size:
                     webp_path.parent.mkdir(parents=True, exist_ok=True)
                     with open(webp_path, "wb") as f:
@@ -283,10 +260,11 @@ def process_single_image(
         else:
             q = best_params["quality"]
             logger.info(f"Found best quality={q} for {img_path}")
-            psnr_val, compressed_size, _ = measure_webp_quality_pil(original_bgr, pil_image, q)
+            psnr_val, compressed_size, _ = measure_webp_pil(original_bgr, pil_image, quality=q, lossless=False)
             if compressed_size < original_size:
                 webp_path.parent.mkdir(parents=True, exist_ok=True)
                 with open(webp_path, "wb") as f:
+
                     icc_profile = pil_image.info.get("icc_profile", None)
                     # quality 모드로 저장 (원본 모드 유지)
                     pil_image.save(
@@ -338,8 +316,9 @@ def save(
     valid_exts = {".jpg", ".jpeg", ".png", ".bmp", ".tif", ".tiff", ".webp", ".avif"}
 
     # Check extension is same with codec
-    if dst_path.suffix.lower() != f".{codec}":
-        raise ValueError(f"Codec and target extension are not same. {dst_path.suffix} != {codec}")
+    if dst_path.suffix and not dst_path.is_dir():
+        if dst_path.suffix.lower() != f".{codec}":
+            raise ValueError(f"Codec and target extension are not same. {dst_path.suffix} != {codec}")
 
     # Add extension check and warning
     if dst_path.suffix and dst_path.suffix.lower() in valid_exts and dst_path.suffix.lower() not in ['.webp', '.avif']: