On Thu, 20 Sep 2007 06:22:14 -0600, Alex Feldman <[EMAIL PROTECTED]> wrote:
> This is getting off the topic of the Gimp, but you've piqued my
> interest.  I just took a digital photo and modified it very slightly
> with the Gimp, and used exiftool to print out the exif data for the
> original and the modification, and diff'd the two exif outputs.  The
> only things I saw that might have made a difference were fields called
> "Y Cb Cr Sub Sampling" and "JFIF Version".  I don't know what these are,
> but neither one screamed Gimp!, at least to me.  Is there an exif field
> I am missing?  Is there another tool for looking at all this data?

Using exiftool will only give you a part of the EXIF information, and
EXIF is only a part of the metadata available in the JPEG file.  You
will get a lot more information about the layout of the various blocks
contained in the file by using exifprobe instead of exiftool.

Although exifprobe shows much more than the EXIF metadata, it does
not show you an additional bit of info that can be useful when trying
to identify forgeries: what software has created the JPEG quantization
tables used in that file.  In case you are not familiar with JPEG
compression, these tables define how the luminance and chrominance
components of the image are compressed.  Most cameras use their own
tables, Adobe uses their own tables in Photoshop and other products,
GIMP uses the IJG tables, etc.

That's why I suggested using the jpegqual test tool that I included in
the GIMP source tree (only in SVN for the moment).  That program
allows you to check the quantization tables used in a JPEG file and
guess what program or device could have created them.  This program is
very incomplete and not intended for general distribution (I wrote it
to validate some algorithms used in the jpeg save plug-in), but you
can already have some fun with it.

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