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Allo all. I'm about to embark on a pretty large project documenting a large, historical cellar - this has a couple of vertical layers to it, one accessed via narrow stairs, the other by a steep ladder. Would photosynth realise that they're related, and is there anything I should keep in mind to try to facilitate such? If not, I suppose I could just have a few separate synths for each floor, but that's not really ideal.
The other issue here is that only one floor has decent lighting, the rest is torchlit only. Will that add complications, and what should I do to try and avoid problems?
I'll leave it to the photographers proper to suggest how to handle the torchlit floors so that your exposures are decent.
Is the staircase a spiral staircase?
Is the cellar cylindrical?
Is it more of an open shaft or is each floor able to be walked all the way around with only a small hole to reach upper or lower floors?
The absolutely most important thing that you can do in any synth to make sure that it all becomes one integrated pointcloud|photo cluster is to take extra extra care (read: more photos!) whenever passing by large occluders (read: cellar floors, large trees, passing by a building when moving from the alley on one side to the open street on the other when they both open onto the same open square - anything that is going to block out everything you just shot while you move past|around it) as well as whenever turning around (pivoting in place).
Other than that, there should be plenty of unique texture for Photosynth to love. Just be sure to get good crisp photos. A tripod is probably going to be really key for those torchlit floors unless you have a lot of batteries to run your flash continually.
If it were me, I'd want to make a good continuous sweep of photos covering each wall and a nice clean orbit in each corner of each floor. If there are supporting pillars or anything of that sort in the middle of the floor, I'd want to orbit those as well.
Regarding connecting one floor to the next, there are at least two techniques that come to mind.
:: The first is to keep the camera trained on the *opening* to the floor below as you descend, continuing to track the divide as you pass below it until you are standing on the next floor. If it can be safely walked, I would even try circling the opening to the floor above which you've just come through after you've reached the new floor before looking around the new environment.
:: The second would be possible if the floor below can be lit with your camera's flash or is bright enough without the flash for your camera to pick up good texture from the floor ahead to track from your current floor all the way down the stairs or ladder until you're actually standing on that part of the floor. If you're ascending then this applies to the ceiling rather than the floor. This technique also relies on you having defined the opening to the next floor from your current floor.
The key to both techniques is to pick a point and track it very vigorously when the scenery is about to change dramatically - whether that is the stairs or ladder, floor or ceiling, or the mouth of the passage of one floor to the next, I leave to your discretion.
It may seem strange to you, but if you have the available memory on your memory cards, a single circle of photography over a portion of floor or ceiling should immediately light up that part of the model when Photosynth reconstructs the scene, provided that it can recognise some of the walls that you've already shot.
If you haven't seen it already, I highly recommend studying LostInTheTriangle's legendary "Martello Tower: Inside & Out":
Tap the [P] key twice after you click within the synth to see the pointcloud and give it a moment to load in. His pointcloud is amazing, but you can easily see where the top floor could have used some shots of the ceiling. The lower floor with the domed ceiling, he captured perfectly and the staircases are a thing a beauty.
His one criticism was that the inner wall and outer wall were not as close as they should have been. I think that had he orbited the sides of the doorways from inside to out, he could tighten the alignment in the same way that I am suggesting that you track the opening between floors as you move from one to the next.
Again, granted that you have more space than you know what to do with on your memory cards it's always better to shoot more than you need or can fit into one synth and be able to choose what you don't need than to find that to truly get the entire environment to unite, you'll need to make another trip.
Two excellent exercises for teaching yourself the value of transitional photos are to try to make a synth of a semi-famous building such as an author's home which has become a museum open to the public using only photos published on photo sharing sites and the internet at large. You will very quickly realise how important it is to have photos moving from one room to the others and even looking from one wall to the others in the same room.
The second exercise is simply to make a synth that connects at least three different rooms in your own house. Mastering this is good training. The walls of the cellar should be more synthy than your house.