Re: fixing PNG images

[Werner LEMBERG]

>> Note that libpng comes with a tool called `png-fix-too-far-back' to
>> fix such errors in PNG files.  Is it OK to update all PNG files in
>> the lilypond repository?  I've been able to successfully continue
>> compilation with the current SVN TeXlive binaries after running
>> this program on all PNG images.
> 
> How did the problematic PNG files come about?

This is a good question.  IIRC, the libpng people say that a
yet-to-be-identified program or library creates such faulty PNG
files.  From the manual:

  Libpng-1.6.0 and later use the CMF bytes at the beginning of the
  IDAT stream to set the size of the sliding window for reading
  instead of using the default 32-kbyte sliding window size.  It was
  discovered that there are hundreds of PNG files in the wild that
  have incorrect CMF bytes that cause libpng to now issue a "too far
  back" error and reject the file.  Libpng-1.6.3 provides a way to
  revert to the libpng-1.5.x behavior (ignoring the CMF bytes and
  using a 32-kbyte sliding window), and provides a tool
  (contrib/tools/png-fix-too-far-back) for optimizing the CMF bytes
  correctly.


    Werner