Merge cba716420a into 877527cefc

Tests/test_file_webp_lossless.py

@@ -4,7 +4,7 @@ from pathlib import Path
 
 import pytest
 
-from PIL import Image
+from PIL import Image, WebPImagePlugin
 
 from .helper import assert_image_equal, hopper
 
@@ -26,3 +26,21 @@ def test_write_lossless_rgb(tmp_path: Path) -> None:
         image.load()
 
         assert_image_equal(image, hopper(RGB_MODE))
+
+
+def test_is_lossless(tmp_path: Path) -> None:
+    lossless_file = tmp_path / "lossless.webp"
+    hopper(RGB_MODE).save(lossless_file, lossless=True)
+    with Image.open(lossless_file) as image:
+        assert isinstance(image, WebPImagePlugin.WebPImageFile)
+        assert image.is_lossless is True
+
+    lossy_file = tmp_path / "lossy.webp"
+    hopper(RGB_MODE).save(lossy_file, lossless=False)
+    with Image.open(lossy_file) as image:
+        assert isinstance(image, WebPImagePlugin.WebPImageFile)
+        assert image.is_lossless is False
+
+    with Image.open("Tests/images/hopper.webp") as image:
+        assert isinstance(image, WebPImagePlugin.WebPImageFile)
+        assert image.is_lossless is False

docs/handbook/image-file-formats.rst

@@ -1375,6 +1375,13 @@ WebP
 
 Pillow reads and writes WebP files. Requires libwebp v0.5.0 or later.
 
+The :py:class:`~PIL.WebPImagePlugin.WebPImageFile` class also exposes the
+following attribute:
+
+**is_lossless**
+    ``True`` if every coded frame in the file uses VP8L (lossless)
+    compression, ``False`` otherwise.
+
 .. _webp-saving:
 
 Saving

src/PIL/WebPImagePlugin.py

@@ -22,6 +22,38 @@ _VP8_MODES_BY_IDENTIFIER = {
 }
 
 
+def _is_lossless(data: bytes) -> bool:
+    # A WebP file is considered lossless when every coded frame uses the
+    # VP8L bitstream. See https://developers.google.com/speed/webp/docs/riff_container
+    chunk = data[12:16]
+    if chunk == b"VP8L":
+        return True
+    if chunk != b"VP8X":
+        return False
+
+    # Extended file format: walk the sub-chunks looking for any lossy frame.
+    pos = 12
+    found_frame = False
+    end = len(data)
+    while pos + 8 <= end:
+        fourcc = data[pos : pos + 4]
+        size = int.from_bytes(data[pos + 4 : pos + 8], "little")
+        pos += 8
+        if fourcc == b"VP8 ":
+            return False
+        if fourcc == b"VP8L":
+            found_frame = True
+        elif fourcc == b"ANMF" and pos + 20 <= end:
+            # ANMF: 16 bytes of frame info, then one VP8 / VP8L sub-chunk
+            sub = data[pos + 16 : pos + 20]
+            if sub == b"VP8 ":
+                return False
+            if sub == b"VP8L":
+                found_frame = True
+        pos += size + (size & 1)  # RIFF chunks are padded to an even length
+    return found_frame
+
+
 def _accept(prefix: bytes) -> bool | str:
     is_riff_file_format = prefix.startswith(b"RIFF")
     is_webp_file = prefix[8:12] == b"WEBP"
@@ -51,6 +83,7 @@ class WebPImageFile(ImageFile.ImageFile):
 
         # Use the newer AnimDecoder API to parse the (possibly) animated file,
         # and access muxed chunks like ICC/EXIF/XMP.
+        self.is_lossless = _is_lossless(s)
         self._decoder = _webp.WebPAnimDecoder(s)
 
         # Get info from decoder