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I was wondering if the background is used for the calculations, because sometimes it is easier to rotate the object instead of the camera for Spins, but then the movement happens only on the object and not on the background and this could affect the motion detection. Is this possible, to rotate the object instead of the camera to get the same results?
At first I thought you were asking about the viewer rather than when you were shooting.
One of the Frequently Asked Questions addresses this a bit. http://photosynth.net/preview/help#FAQ14
The Expert Shooting Guide also mentions it at the end of 'Spin Tips'
(begins at the end of Page 3, but this quote is from the top of Page 4, just before 'Wall Tips'):
The rule of thumb for turntables is to make anything that doesn’t rotate disappear.
In practice, this
means that any nonrotating background needs to be as featureless as possible."
Unless there is some major difference in how the new Photosynth computes Spins from how it computed Objects in the original synths, it is tracking all identified feature points. This means that it is tracking foreground + background so if your background has lots of unique features, those will match more than the turning object's.
The fundamental question that Photosynth is trying to answer is, "How far away from each other are all observed points in all photos relative to each other?".
Because of this it (historically) can only reconstruct parts of a scene that are rigid and stationary.
Over the years I've seen many people try turntable type shots where they turn the object rather than rotating the camera around the subject, but unless your backdrop is almost totally blank Photosynth will conclude that the background has much more in common between photos than the moving object.
Take a look at David Gedye's synth of the wrapped rocks, currently featured on the homepage, you can see a good example of a suitably blank backdrop: http://photosynth.net/preview/view/d3681aca-984c-44aa-8dd0-9660d1620ca4
(His description on the synth points you to read the guide.)
I created this one (originally for an old Photosynth, but I used the same image files to create a new spin) by putting the object on a turntable and spinning that:
However, I used a very plain background, and many images that showed the background were rejected.
(here's the original: http://photosynth.net/view.aspx?cid=f0c90b2d-9176-481f-883a-f32472ad6e6e