New Geotag extension

[Eitan Isaacson <eitan ascender com>]

Hi.

Last week I was itching to get geotagging support into f-spot.
So I spent some time over the weekend getting an extension started that
would correlate GPS track data with photo timestamps, and write the
coordinates into the jpeg's exif header.

It's in a sloppy SVN repo:
http://monotonous.org/geotag

This is the workflow in the current version:
- Open the geotag tool from the Tools menu.
- Load a GPX track file in the geotag window (only tested with
maemo-mapper tracks).
- The main photo view will narrow down to the photos taken during the
track's timestamps.
- Drag photos you want to geotag in to the treeview.
- Watch the view populate with thumbnails and coordinates.
- You could preview the position of any given photo by selecting it and
clicking "show on map" - it will bring up a pushpin in google maps in
your browser.
- Press ok. And wait patiently for your photos to be tagged.
- Upload to flickr, and share your map.

To do:
- Tag multiple versions, now only the default version gets tagged (maybe
that is enough?).
- Deal with RAW. I didn't even test this, or for that matter anything
non-jpeg.
- Allow timestamp adjustments in the dialog. My Canon digital rebel does
not provide a way to synchronize the clock very well, so the timestamp
will never be very accurate (I heard you could do that in Nikon, but
don't get me started!).

Far future:
- Include a map widget. Unless you are a dork like me walking around
with a GPS logger, this extension is useless. A map widget will allow
people to review the coordinate info, and easily adjust it. I could
personally live without the map now, because I upload pics to flickr,
and it does the pretty map bit for me.

Here is another list, I could think of 4 uses for such an extension,
maybe they should actually be separate ones:
- Easily geotag a photo (using a map).
- Correlate GPS info and batch-tag photos (this is what this does
already).
- Review photo collection in a geospatial manner, like the date glass
above, but instead of time, geographic location. We need a map for this
too.
- Auto-tag photos by their geotag, like "Alexandria, Egypt".

Oh, and here is another list of issues I ran into with f-spot, I need to
make bug reports out of them, unless I hear otherwise.
- There is no way of having a singleton instance of an extension. It
seems like there is no place to store such an instance globally.
- Tiff.cs/Exif.cs: It took me an hour to figure out which library is
coming, and which is going. I think the idea is to get rid of Exif,
right? Anyway, I saw no good examples on how to write data wit Tiff, so
I stuck to Exif, which is lacking in the GPS directory department, so I
had to include a few hacks.
- F-spot over writes datetimeoriginal, I don't think this is wise. Every
time f-spot imports a photo, it reads datetimeoriginal, and pushes the
timestamp to what it thinks is UTC. The problem is that if you import a
photo, and then re-import the same file on you next f-spot installation,
it pushes in further past UTC. So say you are in GMT+2, and the photo
was taken at 16:00 local time, f-spot will normalize it to 18:00, and
then in the next import it will be 20:00.

Cheers,
	Eitan.