Posts from: May '08

Time sync with AGG

I decided to write a short article on how to use the Time Offset feature in my resizer/gallery generator AGG. The thing that pushed me to this: I recently went on a three day trip to Vienna with some friends, and we combined the photos from several cameras, summing up to the astonishing 1700+ images. Even I was doubtful whether AGG would cope with handling that amount of photos and the task of arranging them for comfortable viewing. And there are quite a few problems that crop out when sorting photos chronologically:

1. When you sort the photos by name, one usually pays more attention to the first set of photos (1st camera), and after that he hastily jumps through the others, which are "almost the same" and "déjà vu".
2. Sorting by Exif date (as initially proposed by singularity from HardwareBG boards) seldom works. Seems that people don't care much about correct clock settings in their cameras - about 3/4 of the cases you got the correct time shifted by an hour, or a day, or even a few years. And, sometimes, it is never changed from factory settings and reflects Some Chineze Time (tm).
3. "Time Offset" from AGG lets you choose a few photos, taken at "approximately the same time", and corrects the individual cameras' clocks in order to match the date/time of the "coinciding" photos. Sounds good on paper, but in practice, "approximately the same time" doesn't quite cut it - you need "exactly the same time" - even a few seconds are problematic

The Solution:
In Vienna, I invented the following work flow, which turned out to perform surprisingly well:
1. I convinced the people that time synchronization is important, and we did a group shot (less than 1s apart) of one specific thing (my watch in this case).
2. ... But one of the guys (Tisho) wasn't there when we did it, so we had to make another shot "together" with mine and his camera only.
Note: The synchronization procedure works if the following is fulfilled: If we represent a camera with a vertex in a graph, and if a "shot together" of two cameras is represented as an edge between the corresponding graph vertices, then the collection of photos is synchronizeable iff graph is connected.
3. So, when I got all the photos on the hard drive, my first job was to sync with Tisho. I added our two photos into the Time Offset list, my camera first, then his. Then I double-clicked on his entry in the Time Offset list. AGG calculated the time delta against me - I wrote it down on paper.
4. Then, I cleared the Time Offset list and found all the images of my watch (the large group photo), and added my camera as first, and the others' - in an arbitrary order.
5. Since Tisho wasn't in the group shot, but I already had the correct time delta against him, I added an arbitrary photo from his camera (time delta would be changed anyway) and, after a doubleclick on his entry in the list, I entered the time delta I got saved earlier.
6. After all this, I clicked on the "Sort" button and the gallery was ready.

Posted in category Open source -- 23 May 2008, 18:04, 0 comments

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