Re: [png-mng-implement] issue with libpng 1.5.10, ExifTool and text chunks

[Glenn Randers-P. <gl...@gm...>]

I received a copy of the PNG "ringo4.png" and
pngcrush -n -v sees the added zTXt and iTXt
chunks.  pngcrush was built with libpng-1.5.10

Glenn

On Tue, Jun 19, 2012 at 12:21 PM, Andreas Kleinert
<And...@gm...> wrote:
> Using a command line like the following, ExifTool (here: tried v7.77 and
> v8.95, i.a. latest) is able to transfer meta data from JFIF (here: Exif,
> Iptc and XMP chunks) to PNG using a command line like the following:
>
>        exiftool -tagsfromfile some.jfif some-existing.png
>
> The PNG file gets modified (after being backed up) and at its very end
> some (here: 3) text chunks are added, which carry that additional meta
> data. These chunks are (here: 3 in that order):
>
>        zTXT (Exif data)
>        zTXT (Iptc data)
>        iTXT (XMP data)

The file actually contains zTXt and iTXt which is correct.

>
> ...and then IEND follows.
>
> Now the issue. If I try to read such a file into my own reader(*),
> png_get_text() never reports back any of these text chunks.
>
> If I, however, use my own writer(*) - which uses png_set_text() - to put
> these chunks initially (instead of ExifTool) they are placed earlier
> into the file and I don't have any issues later when trying to read them
> back again - png_get_text() finds them and I can handle them.

Is your code looking for them in read_ptr or in end_ptr, if you have
separate png_struct pointers for before and after IDAT?


>
> So my conclusion is that my code is basically correct and that libpng
> 1.5.10 either considers the ExifTool-created file malformed (after some
> point) or stops parsing for *txt chunks at some point for some (other)
> reason.
>
> Question:
> - is there any known issue in libpng 1.5.x with "late positioned"
>  text chunks?
> - is there any known issue in ExifTool in adding these wrongly?
>
> I thought I'd post that here first, as the PNG file does not seem
> corrupted and as ExifTool itself seems to be able to read back
> data from both kinds of output files.
>
>
> (*) SView5, just for the record
>
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