Re: [Libexif-devel] Writing EXIF back to JPEG file

[Dan F. <da...@co...>]

On Fri, Apr 03, 2009 at 01:32:19PM -0700, Jim Nelson wrote:
> I'm trying to write EXIF data back to the JPEG file it came from.  This appears
> to be a problem for a number of people.  If there's a general practice for this
> -- or available code! -- I'd love to know where to find it.

There used to be a little library called libjpeg that was a part of libexif,
but it has since been moved to the exif command-line tool source package.
You can easily copy that out and use it in your package, being mindful
of the license, of course.

> My own experiments show that libexif is, for at least the photos I'm using,
> reducing the size of the EXIF data by approx. 1800 - 2000 bytes, even before my
> modifications.  I assume this is due to padding in the original JPEG file that
> libexif bit-buckets at load time.

That's likely. In some cases libexif will remove malformed tags that it
runs into, which may be another reason for a size decrease.

> Here's a general strategy I'm considering:
> 
> 1. Read EXIF from the JPEG.  Modify.
> 2. Flatten into buffer with exif_data_save_data().
> 3. Examine JPEG file's APP1 segment.  If APP1 length <= buffer length, write
> buffer directly into the APP1 segment in the file *without* modifying APP1's
> length field.  Zero out remaining bytes in file.  Done.
> 4. Otherwise, write out temporary file with SOI, APP1 with new EXIF and proper
> EXIF length, then copy remaining original file over.  Move temporary file on
> top of original.  Done.
> 
> Step #3 is kind of dodgy ... will this work?  I don't know enough about the
> EXIF or JPEG spec to understand if this is a problem (especially using zeroes
> as padding).  I'm not worried about wasting 2000 bytes in a 1MB file.

libjpeg should do what you need in this area.

>>> Dan
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