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Author Topic: Aligning top and bottom of an object  (Read 13842 times)

William Ringle

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Aligning top and bottom of an object
« on: March 09, 2016, 12:00:39 AM »
I would like to align the top and bottom of an artifact, in this case a pot with three legs, to create a full 3D model. The surface has a painted design plus cracks, and the legs are modeled, so the object is not at all a regular or featureless surface.

Rather than create masks (too time consuming for production purposes), my idea was to take the object upside down (42 photos) and create a model, than do the same with the pot right side up, 45 photos. In both cases the photos from the two chunks aligned very well, so I had one chunk resting upright on a flat surface, the other inverted on the flat surface. I made sure to take a circle of photos around the side in both cases.

I had hope that by removing both the dense and sparse clouds of the support layer (the flat surface), plus a bit more, the two chunks would align. They aligned, but with the legs from the inverted model pointing up, and those from the normal model pointing down (see attached).

Can anyone explain the error of my ways and why Photoscan will not align the two correctly (or even why it will align it the way it does!). I thought alignment was performed on the point cloud.

Thanks

Kiesel

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« Last Edit: March 09, 2016, 11:27:35 PM by Kiesel »

James

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Re: Aligning top and bottom of an object
« Reply #2 on: March 09, 2016, 01:06:12 PM »
seems like photoscan is finding a better alignment using points on the flat surface than on the surface of the artifact.

you say you did not use masks, which would help explain that.

you can create masks for your two chunks very easily using tools -> import masks -> from model, asusming you have built meshes for the individual chunks - if not you can do one on low settings as it doesn't need to be great quality for this purpose.

then align chunks by points and make sure you choose constrain features by mask to only align the chunks using points on the artifact surface.

In fact you may get better results by merging the chunks with your imported masks (abandoning any dense cloud or mesh) and aligning the images again using the masks to constrain feature detection, because align chunks does not create tie points between images in different chunks, so although the chunks may align, they might not align very well if one or other is slightly distorted relative to the other.

bigben

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Re: Aligning top and bottom of an object
« Reply #3 on: March 09, 2016, 03:32:45 PM »
The method outlined by James should work fine.  If you try this again.  Use a completely different background (so the backgrounds in the 2 chunks won't align to each other. For the upside down shot, prop the object up so that it isn't touching the background. That should make removal of the background easy.

William Ringle

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Re: Aligning top and bottom of an object
« Reply #4 on: March 09, 2016, 04:20:01 PM »
Thank you all for your replies. I tried James's method and it works great, so now I have a full 3D model with both top and bottom. (BTW, I did not change the background, but will try that tactic for the next go round.)  The workflow blog referred to by Kiesel is also admirably clear, so thanks for that too.

My question still remains, though. How does chunk alignment proceed - sparse or dense point cloud, mesh? Like I mentioned, I removed the supporting surface from both the upright and inverted models in my original attempt, but got that weird alignment. I would have thought having just the point clouds and meshes of the pot itself would have allowed the two chunks to align.

Thanks again for your help.

James

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Re: Aligning top and bottom of an object
« Reply #5 on: March 09, 2016, 05:52:16 PM »
Align chunks by points doesn't use 'points' of the point cloud, but detected feature 'points' in the photos, so whatever you do with the point cloud makes no difference.

sc

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Re: Aligning top and bottom of an object
« Reply #6 on: March 09, 2016, 07:51:45 PM »
If you mask all your photos, you shouldn't have any problems aligning your chunks

JMR

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Re: Aligning top and bottom of an object
« Reply #7 on: March 09, 2016, 08:19:31 PM »

F.Tella

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Re: Aligning top and bottom of an object
« Reply #8 on: July 08, 2016, 02:08:12 AM »
Hi, I guess the procedures explained in these tutorials can only be achieved using pro version, right? Because I can't find how to make markers in my version.

So my only option goes through masking the pictures then as I only have Standard version, is this correct?

Thanks!
Fernando

William Ringle

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Re: Aligning top and bottom of an object
« Reply #9 on: July 08, 2016, 02:53:25 AM »
I believe markers are a feature of the Pro version only. But applying masks is quite rapid once you have the top and bottom models created. Just strip away the material you don't want first.