Forum : Advice & Critiques

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Topic: Debug Approach

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clkotnik (7 months ago)
I have just started the Photosynth 2014 Tec Preview and after some quick and initially successful synths, tried what I thought would be a sure thing.  I put a model on a revolving chair and found that something really confused the processor.  
http://photosynth.net/preview/view/44daa969-0190-479e-a49f-db5e6a28d070

So now I'm trying to figure out what went wrong. I suspect my background was too textured, but it makes me wonder what debugging techniques folks have found since I'm sure I'll make other mistakes.

I found a document call the Photosynth Photography Guide which mentions viewer keyboard shortcuts and the point cloud which sounds like it might help me.  However only a fraction of these work for me (Win 7 IE 11) and I cannot find the point cloud.  

Any suggest would be welcome.
NateLawrence (7 months ago)
Hi there, clkotnik, 

Original photosynths in Photosynth 1 were a lot more free form (in that you could combine spins, walks, walls, and panoramas in a single synth) and were all about showing the photos in the context of the point cloud of the scene. I liked that very much and miss it in Photosynth 2.

In Photosynth 1, though, each photo was only projected onto a single flat rectangle (instead of projecting each photo onto a low polygon model like in Photosynth 2) and each photo was only loaded after you moved to look at it (instead of loading a medium resolution of all the photos as fast as possible ahead of time like Photosynth 2), which meant that the playback experience of viewing spins, walks, walls, + panorama style synths in Photosynth 1 wasn't quite as compelling (at least in those areas).
NateLawrence (7 months ago)
You were right in your diagnosis when you said, "I suspect my background was too textured.". Here's someone else who had exactly your problem just this past weekend.
http://photosynth.net/discussion.aspx?cat=00581351-82d8-438d-a37b-7eadb3fb4991&dis=a7ee2de8-0d79-46fa-bf43-5da66e7ae5f7

For any other 'Photosynth 2' / 'Photosynth 2014 Technical Preview' questions, here's the forum dedicated to it: 
http://photosynth.net/forum.aspx?cat=00581351-82d8-438d-a37b-7eadb3fb4991

When you reference the 'Photosynth Photography Guide' available at http://photosynth.net/help.aspx#photosynthhelp that is actually the Photosynth 1 shooting guide and although there are similarities (especially around shooting turntable spins like you're wanting to), you'll want to get the Photosynth 2 shooting guide or 'Expert Shooting Guide' here: http://photosynth.net/preview/help
NateLawrence (7 months ago)
In either Photosynth 1 or Photosynth 2, though, what Photosynth is trying to do is identify patches of each image in as many of the other images as possible.

What you may not have considered, however, is that the fundamental question that Photosynth is trying to answer is, "How far is each point found in three or more images from other points seen in three or more images, relative to each other?".

What this means is that Photosynth only understands rigid objects that are stationary in the scene. That means that you won't get a good point cloud of rippling reflective water of a river, lake, or ocean or a flag waving in the breeze or people who are walking through your shots when you're shooting a walk because there is no single answer to how far any keypoints observed in their part of the photo is to keypoints observed in other parts of the scene.
NateLawrence (7 months ago)
What this means for turntable synths for making 'objects' (as they were called in Photosynth 1) or 'spins' (as they're now called in Photosynth 2) is that if you want Photosynth to track the object you are turning in front of a stationary camera, your backdrop needs to be completely devoid of detail or visual texture.

If this is not the case, then Photosynth will simply track the background through all the shots and offer a reconstruction that favors the background, since it is the most consistent thing found visually and it will treat the spinning foreground object as noise that it couldn't reconstruct - the same as the moving people would get discarded if you were shooting a spin of a public fountain in a town square.
NateLawrence (7 months ago)
The easiest way to shoot a spin is to actually just move the camera around the object you want to make a spin of so that Photosynth can do its work well. 
The question that Photosynth is trying to answer will make sense, because the stationary object really will be a fixed difference from all stationary parts of its surroundings.

If you are dead set on using a stationary camera (say on a tripod, focused on a spinning chair, turntable, etc.) then you'll want one of two approaches: 
1: Use your camera's exposure settings to overexpose or underexpose the background so that all detail is lost there and Photosynth can focus on the foreground.
2: Use an image editor before feeding the photos to Photosynth to erase the background or darken/lighten it enough that there are no remaining details for Photosynth to find.

Sometimes, people have built their own inverse turntable, where the camera is mounted on an arm that swings around the platform that their object sits on.
NateLawrence (7 months ago)
I'm making an updated list of Photosynth 2 keyboard shortcuts here as they're put in and announced which you can find here: 
http://photosynth.net/discussion.aspx?cat=00581351-82d8-438d-a37b-7eadb3fb4991&dis=c120f279-4c92-49af-8ff6-27e62750addb

If you have any more questions, please do ask and the rest of us (or the Photosynth team) will do our best to get you an answer.

Your fellow synther,
N
clkotnik (7 months ago)
Thank you so much for the information.  I see more useful info in the tech preview forum.

And of course the ultimate answer is, as always, 42.
NateLawrence (7 months ago)
My pleasure. ツ

Looking forward to seeing more from you.
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