[gdal-dev] libpng and autotest

[Jack Howarth]

On Fri, Nov 08, 2013 at 09:12:05AM -0800, Kurt Schwehr wrote:
> Jack,
> 
> Is there an explanation someplace with more detail as to what is wrong those png's, why older versions of libpng are okay with it and why libpng has changed its behavior?
> 
> Thanks,
> -kurt

Kurt,
   The following link...

http://comments.gmane.org/gmane.comp.graphics.png.devel/6017

appears to discuss the issue we are seeing with the test.png file...

>  Libpng-1.6.0 through 1.6.2 use the CMF bytes at the beginning of an zlib datastream to set the window size for decoding.  
>  Libpng-1.5.x and earlier ignored the CMF bytes and always used a 32kbyte window.  Libpng-1.6.x has uncovered the fact 
>  that there are hundreds of files in the wild (found in linux distributions such as gentoo) with a too-small CMF, which 
>  results in a "Too far back" error when decoding these PNGs.  I have not yet been able to discover what application is producing these.

If you read completely through the gmane.comp.graphics.png.devel/6017 thread, you should be
at least convinced that such corruption is a known issue.
          Jack

> 
> 
> 
> On Nov 8, 2013, at 8:02 AM, Jack Howarth <howarth at bromo.med.uc.edu> wrote:
> 
> > On Fri, Nov 08, 2013 at 10:31:08AM -0500, Jack Howarth wrote:
> >> On Thu, Nov 07, 2013 at 04:34:07PM -0500, Jack Howarth wrote:
> >>> The failures on x86_64-apple-darwin13 for the png.py test suite from auto
> >>> test for gdal 0.10.1 are the same failures described here...
> >>> 
> >>> https://github.com/licq-im/licq/pull/32
> >>> <https://github.com/licq-im/licq/pull/32>
> >>> 
> >>> The failures are completely suppressed by regenerating
> >>> autotest/gdrivers/data/test.png with
> >>> "optipng -nb -nc -np test.png"...
> >> 
> >> Just to clarify, I used optipng 0.6.3 for this. The newer optipng 0.7.4
> >> release seems more problematic as none of the obvious combination of options
> >> can fix the test.png file without changing the checksums and causing additional
> >> failures. So you will want to use the optipng 0.6.x release series to fix
> >> the bad png files in gdal.
> > 
> > Also, my tests here using optipng 0.7.4 to detect damaged png files indicate
> > that only data/gdalicon.png in gdal-0.10.1 needs to be fixed.
> > 
> > optipng -nb -nc -np -fix data/gdalicon.png
> > ** Processing: data/gdalicon.png
> > Warning: 
> > 32x32 pixels, 4x8 bits/pixel, RGB+alpha
> > Recoverable errors found in input. Fixing...
> > Input IDAT size = 1964 bytes
> > Input file size = 2021 bytes
> > 
> > Trying:
> >  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 240
> > 
> > Selecting parameters:
> >  zc = 9  zm = 8  zs = 0  f = 0		IDAT size = 240
> > 
> > Output IDAT size = 240 bytes (1724 bytes decrease)
> > Output file size = 313 bytes (1708 bytes = 84.51% decrease)
> > 
> > ** Status report
> > 1 file(s) have been processed.
> > 1 error(s) have been encountered.
> > 1 erroneous file(s) have been fixed.
> > 
> > 
> >> 
> >>> 
> >>> % ./png.py
> >>>  TEST: png_1 ... success
> >>>  TEST: png_2 ... success
> >>>  TEST: png_3 ... success
> >>>  TEST: png_4 ... success
> >>>  TEST: png_5 ... success
> >>>  TEST: png_6 ... success
> >>>  TEST: png_7 ... success
> >>>  TEST: png_8 ... success
> >>>  TEST: png_9 ... success
> >>>  TEST: png_10 ... success
> >>>  TEST: png_11 ... success
> >>> 
> >>> Test Script: png
> >>> Succeeded: 11
> >>> Failed:    0 (0 blew exceptions)
> >>> Skipped:   0
> >>> Expected fail:0
> >>> Duration:  0.08s
> >>> 
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