Fix ImImagePlugin.seek() frame stride for non-8-bit modes

seek() computed the per-frame byte stride as 8 * len(self.mode), using
the length of the mode *name* string as bits-per-pixel. That is only
correct when len(mode) equals the band count and the bands are 8-bit
(L, RGB, RGBA, CMYK, LA, P). For I;16 (16-bit), I / F (32-bit) and
YCbCr (3-byte) the stride was wrong, so seeking to frame >= 1 in a
multi-frame IM image either raised 'image file is truncated' or returned
pixels from the wrong offset.

Use 8 * bands * itemsize, which matches Image.new(mode).tobytes() for
every IM-supported mode and leaves the working modes unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Andrew Chen <48723787+chuenchen309@users.noreply.github.com>

Author: chuenchen309

Tests/test_file_im.py

@@ -1,14 +1,15 @@
 from __future__ import annotations
 
 import filecmp
+import io
 import warnings
 from pathlib import Path
 
 import pytest
 
 from PIL import Image, ImImagePlugin
 
-from .helper import assert_image_equal_tofile, hopper, is_pypy
+from .helper import assert_image_equal, assert_image_equal_tofile, hopper, is_pypy
 
 # sample im
 TEST_IM = "Tests/images/hopper.im"
@@ -83,6 +84,41 @@ def test_eoferror() -> None:
         im.seek(n_frames - 1)
 
 
+@pytest.mark.parametrize(
+    "image_type, rawmode, mode",
+    (
+        ("L 16", "I;16", "I;16"),
+        ("L 32S", "I;32S", "I"),
+        ("L 32F", "F;32F", "F"),
+        ("YCC", "YCbCr;L", "YCbCr"),
+    ),
+)
+def test_seek_non_8bit(image_type: str, rawmode: str, mode: str) -> None:
+    # The frame stride must be derived from the actual bytes per pixel, not
+    # from the length of the mode name (which only matches for 8-bit bands).
+    w, h = 4, 4
+    frames = []
+    for base in (0, 1000):
+        frame = Image.new(mode, (w, h))
+        for y in range(h):
+            for x in range(w):
+                frame.putpixel((x, y), base + y * w + x)
+        frames.append(frame)
+
+    header = (
+        f"Image type: {image_type} image\r\n"
+        f"Image size (x*y): {w}*{h}\r\n"
+        f"File size (no of images): {len(frames)}\r\n"
+    ).encode("ascii")
+    header += b"\x00" * (511 - len(header)) + b"\x1a"
+    data = b"".join(frame.tobytes("raw", rawmode, 0, -1) for frame in frames)
+
+    with Image.open(io.BytesIO(header + data)) as im:
+        for index, frame in enumerate(frames):
+            im.seek(index)
+            assert_image_equal(im, frame)
+
+
 @pytest.mark.parametrize("mode", ("RGB", "P", "PA"))
 def test_roundtrip(mode: str, tmp_path: Path) -> None:
     out = tmp_path / "temp.im"

src/PIL/ImImagePlugin.py

@@ -30,7 +30,7 @@
 import re
 from typing import IO, Any
 
-from . import Image, ImageFile, ImagePalette
+from . import Image, ImageFile, ImageMode, ImagePalette
 from ._util import DeferredError
 
 # --------------------------------------------------------------------
@@ -300,7 +300,8 @@ def seek(self, frame: int) -> None:
         if self.mode == "1":
             bits = 1
         else:
-            bits = 8 * len(self.mode)
+            mode = ImageMode.getmode(self.mode)
+            bits = 8 * len(mode.bands) * int(mode.typestr[-1])
 
         size = ((self.size[0] * bits + 7) // 8) * self.size[1]
         offs = self.__offset + frame * size