[Image-SIG] PNG patch for reading zTXt chunks

Lowell Alleman lowell at allemansonline.com
Thu Feb 22 22:36:37 CET 2007

Here is a patch to add support for reading compressed text chunks
(zTXt) and storing them in the info dictionary.  With this patch, both
compressed and uncompressed (tEXt) text chunks can be accessed uniformly.

I've come across some text tag discussions on this mailing list from
many years ago.  Since then, it appears that some support has been
added, but not for reading zTXt chunks.  Since it has been a while,
I'm going to ask that support for reading zTXt chunks be considered again.

Previous discussions (patches):

Here is my patch.  I borrowed ideas from the two patches listed
above.  It should apply cleanly against the 1.1.6 release.

diff -Naur Imaging-1.1.6/PIL/PngImagePlugin.py Imaging-LCA/PIL/PngImagePlugin.py
--- Imaging-1.1.6/PIL/PngImagePlugin.py 2007-02-21 10:55:03.000000000 -0500
+++ Imaging-LCA/PIL/PngImagePlugin.py   2007-02-21 13:27:15.000000000 -0500
@@ -254,6 +254,19 @@
              self.im_info[k] = v
          return s

+    def chunk_zTXt(self, pos, len):
+        s = ImageFile._safe_read(self.fp, len)
+        (k, v) = string.split(s, "\0", 1)
+        comp_method = ord(v[0])
+        v = v[1:]
+        if comp_method != 0:
+            raise SyntaxError, "Unknown compression method %i in zTXt chunk" % comp_method
+        import zlib
+        v = zlib.decompress(v)
+        self.im_info[k] = v
+        return s

  # --------------------------------------------------------------------
  # PNG reader


- Lowell Alleman

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