#65: Store depth information in PNG images themselves.

1 files changed, 58 insertions, 0 deletions

diff --git a/sphinx/util/png.py b/sphinx/util/png.py
new file mode 100644
index 000000000..a22acba2e
--- /dev/null
+++ b/sphinx/util/png.py
@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+"""
+    sphinx.util.png
+    ~~~~~~~~~~~~~~~
+
+    PNG image manipulation helpers.
+
+    :copyright: 2008 by Georg Brandl.
+    :license: BSD.
+"""
+
+import struct
+import binascii
+
+LEN_IEND = 12
+LEN_DEPTH = 22
+
+DEPTH_CHUNK_LEN = struct.pack('!i', 10)
+DEPTH_CHUNK_START = 'tEXtDepth\x00'
+IEND_CHUNK = '\x00\x00\x00\x00IEND\xAE\x42\x60\x82'
+
+
+def read_png_depth(filename):
+    """
+    Read the special tEXt chunk indicating the depth from a PNG file.
+    """
+    result = None
+    f = open(filename, 'rb')
+    try:
+        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
+        result = struct.unpack('!i', depthchunk[14:18])[0]
+    finally:
+        f.close()
+    return result
+
+
+def write_png_depth(filename, depth):
+    """
+    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)
+    f = open(filename, 'r+b')
+    try:
+        # 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
+        f.write(struct.pack('!i', binascii.crc32(DEPTH_CHUNK_START + data)))
+        # replace the IEND chunk
+        f.write(IEND_CHUNK)
+    finally:
+        f.close()