[Image-SIG] PNG patch for reading zTXt chunks

[Lowell Alleman]

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):
       http://mail.python.org/pipermail/image-sig/1998-July/000529.html
       http://mail.python.org/pipermail/image-sig/1999-May/000737.html

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


Thanks,

- Lowell Alleman

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