1 files changed, 43 insertions, 0 deletions
diff --git a/sphinx/util/png.py b/sphinx/util/png.py
new file mode 100644
index 0000000..6c94219
--- /dev/null
+++ b/sphinx/util/png.py
@@ -0,0 +1,43 @@
+"""PNG image manipulation helpers."""
+
+from __future__ import annotations
+
+import binascii
+import struct
+
+LEN_IEND = 12
+LEN_DEPTH = 22
+
+DEPTH_CHUNK_LEN = struct.pack('!i', 10)
+DEPTH_CHUNK_START = b'tEXtDepth\x00'
+IEND_CHUNK = b'\x00\x00\x00\x00IEND\xAE\x42\x60\x82'
+
+
+def read_png_depth(filename: str) -> int | None:
+    """Read the special tEXt chunk indicating the depth from a PNG file."""
+    with open(filename, 'rb') as f:
+        f.seek(- (LEN_IEND + LEN_DEPTH), 2)
+        depthchunk = f.read(LEN_DEPTH)
+        if not depthchunk.startswith(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START):
+            # either not a PNG file or not containing the depth chunk
+            return None
+        else:
+            return struct.unpack('!i', depthchunk[14:18])[0]
+
+
+def write_png_depth(filename: str, depth: int) -> None:
+    """Write the special tEXt chunk indicating the depth to a PNG file.
+
+    The chunk is placed immediately before the special IEND chunk.
+    """
+    data = struct.pack('!i', depth)
+    with open(filename, 'r+b') as f:
+        # seek to the beginning of the IEND chunk
+        f.seek(-LEN_IEND, 2)
+        # overwrite it with the depth chunk
+        f.write(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START + data)
+        # calculate the checksum over chunk name and data
+        crc = binascii.crc32(DEPTH_CHUNK_START + data) & 0xffffffff
+        f.write(struct.pack('!I', crc))
+        # replace the IEND chunk
+        f.write(IEND_CHUNK)