Forum : Advice & Critiques

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Topic: Linking issues on a massive Synth

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jontlaw (Over 1 year ago)
I've extended a previous synth (by creating a new one with added images). The first one was clean, but the new one (616 images)has an enormous twist in the middle, where the linking seems to shift off on an erroneous vector. I've added a highlight that shows the error clearly, but I can't track the images that are causing the problem.

If anyone can give me any pointers as to how to find and fix the issue, I'd be exceptionally grateful!

Thanks in advance.
michaeldenis (Over 1 year ago)
OK, as a someone-not-point-cloud-focused, I agree that your issue is pretty small.  If I understand your issue correctly, you want to show us that the Point Cloud, at the specified area in your synth, diverges completely from what's shown in the photo?  Yes, I agree with that statement.  I tend to find that happens quite a bit, however, because I oftentimes work with large synths.  I notice that many of your previous works tend to have a more finite scale.  Perhaps we've both experienced some of the limits of the large scale synthing and point cloud to image agreements.

By the way, what's in the Regent Square House?  I notice a shift in perspective..

Nice work overall.
jontlaw (Over 1 year ago)
Thanks for the reply. I made note of the point cloud offset as it hi-lights the issue I'm having. The same distortion actually breaks links when viewing the synth alone - some mages are twisted so much that it can't make sense of them. I've added a set of images from across the road (in my 800+ image version!) and this has helped realign the data!

Looking back, I'm thinking that it was probably images that held 50% or more pavement that cause the problem. With a wide lens, there is alot of distortion, and it starts trying to align the pavement plane instead of the vertical buildings. I'm going to go through and crop the pavement off images that show too much or hold massive distortion. There may also be a few close up window and wall shots that will be confused as to their real location due to regular spacing - too many possible links.

Next I'm thinking of adding a series of shots down the center of the street to give a better transition across.
Nathanael (Over 1 year ago)
Hello, all,

Jontlaw, I finally got to sit down with your synth today and take a long look at it. I think that my very first suggestion to you will be very familiar to anyone who has talked with me before. It certainly isn't the only way to get a clean pointcloud, but if you are patient with it, it very rarely fails you.

It is very simply this: Less rotating in place like a panorama; more choosing a particular surface and circling it with your camera. Panoramic shooting isn't all bad, but I would urge more focusing on one object at a time so that you can be sure that you covered each thing from all angles.

In my experience, # of photos is no barrier to a good reconstruction, so long as you have taught Photosynth what each major object looks like from at least 180 degrees (360 degrees when possible) and what it looks like to look from something you've just circled (such as the statues) and then walk over to something else which you've also circled, and so on.
Nathanael (Over 1 year ago)
Any method of shooting will begin to struggle as you spread the synth over more + more distance, but orbiting objects is almost like putting a vice grip on each particular object that doesn't leave Photosynth much room for error.

I don't think that the order that things are shot should make much of a difference as Photosynth eventually compares each photo in a given synth to every other photo in that same synth, so theoretically shooting a bunch of panoramas could get you many similar angles to orbiting, say, the facade of each building individually, but it's leaving more to chance than if you assign a human brain the task of shooting a minimum of 12 photos semicircling the front of each building instead of letting it judge how many more steps before taking another panorama.

Give this method a try + tell me how it goes for you.
Nathanael (Over 1 year ago)
If you'll be trying my approach on a street I think that a long straight line of overlapping photos looking straight at one side of the street at a time, moving from the left corner of one block to the right corner (or right to left if you please) could also help tighten the alignment if walking between each orbit proves to be too many photos.

Perhaps you'll object that all of this smacks of holding Photosynth's hand a little too much... patronizing it, if you will. I would simply say that Photosynth's vision is very much like a child's who has never seen anything before and because of this I give it photos as input which let it move all the way around things so that it can begin to recognize it from all sides and sometimes multiple distances or heights from all sides (if focusing on a smaller area). As of now, Photosynth doesn't gain understanding with each synth made. It isn't 'learning'. Every synth is its first, so proceeding gently is maximally rewarding.
Nathanael (Over 1 year ago)
Unfortunately that doesn't provide you with a quick way to shoot a few more photos and straighten out your existing synth, but the difference that I saw in my pointclouds when I began to prioritise my shooting by object (or individual surface, to be perfectly honest), rather than taking many panoramas close to each other, was quite revealing to my mind as to what was going to work best for building pointclouds, which really began to become my goal rather early on.

Certainly not everyone cares about pointclouds over the photos or the almost absurd number of photos to really draw out fine details in a pointcloud, but I will say this much: To the extent that your pointcloud is well-formed is the extent to which your photos are arranged correctly and nearly everyone should care about that.
jontlaw (Over 1 year ago)
Cheers Nathanael - that's quite an answer :) Thanks for taking the time. I did shoot the other side of the street using panoramas at about 20 foot intervals. This did realign the majority of the pointcloud, and fixed the linking around the dodgy area. Looking back, I think the weak area where it warped off the floorplane had too few shots to compare, and in some of those shots was too much floor, with a strong pattern on it. Rather than aligning the walls of buildings, I think Photosynth started trying to match the floor.

On my next synth I will try the orbit method, see how it compares.I have to say though, I'm pretty impressed with the 800+ shot version of The Parade I did. It's raggedy round the edges, where there are not enough angles on the trees, but still - awesome stuff!
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