Acknowledgements
Before launching into the details here, I'd like to thank somebody who's made it possible to do this work. Olivier Tilloy wrote the python wrapper for the libexiv2 which makes this possible. I've enjoyed email correspondence with Olivier and he has been very encouraging of my efforts.
Andreas Huggel is the author of libexiv2 and of course he's made it possible for Olivier to do his part. I've spoken to Andreas about this and he has also been very helpful.
I believe I haven't needed to even look at a line of code written by Olivier or Andreas - everything just builds and works fine. Well the build isn't a lot of fun - however it does build on Mac OS X, Windows and Linux.
And I've also had some help from Frank and Stani and Anders. Open Source Warriors. Thanks guys.
About the Python Code
The python code's here: click here.
I hope you'll find the code clear and obvious. There are several things which happen:
- I have to guess the delta time between the GPS and the Camera. You can read the comments in the prolog about this. And I discuss this a little further below.
- I read the GPS data from the gpx files into memory. This data is stored in a dictionary. The key of the dictionary is the time (in XML date format). So it's like this:
| Key = XMLTime | : | Value = [ latitude, longitude, elevation] |
- I go over every file in the directory to see if it's an image. If the file was tzinfo or any of the GPX files, then it's ignored. If the file cannot open, it's not an image and is reported. So this pass builds a dictionary of images. The key and values are simply the pathname and it's timestamp. So imagedict is like this:
| Key = XMLTime | : | Value = pathname |
I've did this to ensure that the files are processed in the correct time order. When you inspect the output of the script, it's much easier to spot something amiss (such as an elevation error) when the images appear in the correct time sequence.
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And finally, I go over the dictionary of images files. I obtain the images time stamp and search for the closest GPS location in the timestamp dictionary. The GPS data is then added to the image and saved.
And that's all there really is to it. However there are some little details along the way.
- The GPS time stamp data is an XML date in the format YYYY-mm-ddTHH:MM:SSZ (example today is April 16, 2008 and it's 00:20:45 (just gone midnight in Calfornia). In UTC that's 2008-04-16T07:20:45Z (7 hours ahead of us). The pyexiv2 wrapper uses python datetime to represent time. So we have to convert. The script has to guess the delta between UTC and the Camera. It guesses that this is the local time conversion (taking into account daylight saving). This is recorded in the file tzinfo as seconds. If you rerun the script, it doesn't guess - it reads tzinfo. You could use this to adjust for incorrect time setting of the camera. It also means that a directory of photos will reprocess correctly - even when run on a computer in a different timezone.
- My Garmin records position about every 5 seconds. So the search for the time
I do this using a modified binary search of the timedict. Remember the GPS and Camera might not 'flash' at the same second. The search finds the closestGPS data before (or equal to) the camera time. You could get clever and interpolate the GPS data - however I decided to skip this complication. I usually stop to take photos, so it would make little difference to the position. If you're taking photos from a fast moving vehicle, or with a GPS which records less frequently, you may wish to add GPS interpolation.
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Coordinates in the Exif data are stored as rational numbers (the ratio of two integers). I found the little package surd.py on the internet and I've included it unmodified with gps.py. I found a bug with this. Some elevations were not being handled correctly. I added code to gps.py to ensure that elevation is to the nearest 0.1 metre. This fix is sufficient to correct the elevation issue - however it doesn't address the wider issue that surd.py is creating surds which are not accepted by pyexiv2. I'll fix this sometime in the future.
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Coordinates in the GPS data are stored as ascii strings which represent floating point numbers. I've provided functions to convert back and forward between floats and degress, minutes and seconds for presentation purposes.
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Finally, I've used a little python trick to locate the surd.py package. To save you the inconvenience of putting on your PYTHONPATH, you simply keep it beside gps.py. There's code to modify the module search path by adding the directory which contains gps.py. So you should keep gps.py and surd.py together and put them in a directory on your path.
And that's the story. I hope you find this useful. Send me your bug reports!
Building the libraries on Mac OS X
Separate document: click here.
Building the libraries on Windows
To be written
Building the libraries on Linux
To be written
The build script
This script doesn't build the libraries. It builds the zip file which is published on the web. It was surprisingly difficult to construct and contains some useful lessons.
I wanted to be able to collect all the things together into a zip archive to be published on the web. And I thought "Oh, that can't be difficult". Well it took time to figure out how to mount the Linux correctly from the Mac. And then the script had to be written and tested. So it took a couple of days.
I construct the universal binaries in the build script. And finally, I create the zip file.
The Mac can create zip files from directories directly in finder. This is actually implemented as you would NOT expect by the "ditto" command instead of one of the "gz**" utiliities. Anyway it's done.
Build script: click here. There's another script "release" which actually copies stuff to the web using ftp (and some other details). I've decided not to publish this because it's too boring.
setup.py
I really wanted to create a setup.py script for the release. Then you could simply do:
sudo python setup.py
This would install the stuff and set up the links. However, I haven't done this. I was fed up with the project by this time. If somebody would like to contribute, I'd be very happy to accept their assistance.
One of the difficulties of setup.py is that it has to know where to install gps.py and surd.py on your path. I haven't studied or thought about this puzzle. Maybe the python setup generator has some magic to make this trivial.
Road Map - Phatch
Phatch is an opensource project for a 'Photo Batcher'. It has a library of Action Items such as photo scaling. Actions can be easily sequenced into a work flow to perform processes. Phatch is written in Python - so this seems like the natural home for my code. My intention is to write a Phatch Action Item which uses my code in June, 2008. Assuming it passes testing, this should be available in October 2008.
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