implement tiff exif multistrip support

Tests/test_file_tiff.py

@@ -108,7 +108,8 @@ class TestFileTiff:
             assert_image_equal_tofile(im, "Tests/images/hopper.tif")
 
         with Image.open("Tests/images/hopper_bigtiff.tif") as im:
-            # multistrip support not yet implemented
+            # The data type of this file's StripOffsets tag is LONG8,
+            # which is not yet supported for offset data when saving multiple frames.
             del im.tag_v2[273]
 
             outfile = str(tmp_path / "temp.tif")

Tests/test_file_tiff_metadata.py

@@ -181,6 +181,28 @@ def test_change_stripbytecounts_tag_type(tmp_path: Path) -> None:
         assert reloaded.tag_v2.tagtype[TiffImagePlugin.STRIPBYTECOUNTS] == TiffTags.LONG
 
 
+def test_save_multiple_stripoffsets() -> None:
+    ifd = TiffImagePlugin.ImageFileDirectory_v2()
+    ifd[TiffImagePlugin.STRIPOFFSETS] = (123, 456)
+    assert ifd.tagtype[TiffImagePlugin.STRIPOFFSETS] == TiffTags.LONG
+
+    # all values are in little-endian
+    assert ifd.tobytes() == (
+        # number of tags == 1
+        b"\x01\x00"
+        # tag id (2 bytes), type (2 bytes), count (4 bytes), value (4 bytes)
+        # == 273, 4, 2, 18
+        # == TiffImagePlugin.STRIPOFFSETS, TiffTags.LONG, 2, 18
+        # the value is the index of the tag data
+        b"\x11\x01\x04\x00\x02\x00\x00\x00\x12\x00\x00\x00"
+        # end of tags marker
+        b"\x00\x00\x00\x00"
+        # tag data == (149, 482) == (123 + 26, 456 + 26)
+        # 26 is the number of bytes before this data
+        b"\x95\x00\x00\x00\xe2\x01\x00\x00"
+    )
+
+
 def test_no_duplicate_50741_tag() -> None:
     assert TAG_IDS["MakerNoteSafety"] == 50741
     assert TAG_IDS["BestQualityScale"] == 50780

src/PIL/TiffImagePlugin.py

@@ -991,8 +991,10 @@ class ImageFileDirectory_v2(_IFDv2Base):
         if stripoffsets is not None:
             tag, typ, count, value, data = entries[stripoffsets]
             if data:
-                msg = "multistrip support not yet implemented"
-                raise NotImplementedError(msg)
+                size, handler = self._load_dispatch[typ]
+                values = [val + offset for val in handler(self, data, self.legacy_api)]
+                data = self._write_dispatch[typ](self, *values)
+            else:
                 value = self._pack("L", self._unpack("L", value)[0] + offset)
             entries[stripoffsets] = tag, typ, count, value, data