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Author Topic: What causes these types of reconstruction errors?  (Read 5598 times)

loggie24

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What causes these types of reconstruction errors?
« on: November 25, 2024, 05:54:29 PM »
Hi,

I have noticed some kinds of offset geometry being generated on some of our models. It looks like it creates two layers of tie points and generates two layers of geometry when building the model. Rather tricky to fix in post, does anyone know what could be the issue to either fix this during processing in Metashape or avoid this while capturing the asset?


olihar

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Re: What causes these types of reconstruction errors?
« Reply #1 on: November 25, 2024, 09:56:52 PM »
misalignment will cause this.

loggie24

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Re: What causes these types of reconstruction errors?
« Reply #2 on: November 26, 2024, 02:44:59 PM »
Do you know of any good tricks to fix this without manually placing down control points or check points? One thing I have to some extent been successful with is creating a subject mask to try and mask out unwanted areas and aligning with apply masks to key points, although this has been bit hit and miss. Lowering alignment accuracy also can help but not always. Fundamentally I don't quite understand why these issues happened in the first place as there is a good amount of overlap in the images.
« Last Edit: November 26, 2024, 02:47:17 PM by loggie24 »

olihar

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Re: What causes these types of reconstruction errors?
« Reply #3 on: November 26, 2024, 10:33:11 PM »
Control points will not help in this case.

Tightening up the alignment sounds like the solution here.

Maybe some more info about the dataset.

mraw

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Re: What causes these types of reconstruction errors?
« Reply #4 on: November 27, 2024, 11:38:22 AM »
you have to clean your data before building the model. Use the 'model/gradual selection' tools and 'optimize cameras' back and forth. It's an iterative process because you don't want to filter out valuable information.

loggie24

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Re: What causes these types of reconstruction errors?
« Reply #5 on: November 27, 2024, 09:04:04 PM »
Thanks for the answers fellas.

@mraw I have tried doing this but not much luck to fix these kind of issues, I would almost argue the results are slightly worse after doing this. What I did was:

- Gradual select (reprojection error) 10% of the total amount of tie points, delete
- Optimize cameras with the default settings
- Repeat until I have reprojection error of less than 0.4

I also tried doing similar steps with projection accuracy (down to 3) and reconstruction uncertainty (down to 10).

I'm I missing something or is there a different approach to doing this I'm unaware of?

@olihar What info would you need exactly? It's a scan of a medium scaled broken trunk, so a lot of awkward angles.  Images are of good quality (Sony A7R4 w/cross polarized flash), albeit there is quite a bit of DOF in them due to the awkward shapes of the trunk. There we're two passes done on it as I was verifying the data on location and the first pass showed some poorly covered areas. The second pass is what is causing the alignment problems, it was about 20 min after the first pass. The object had not been moved or touched, nothing really changed hence I'm a bit confused why the second pass caused this.

« Last Edit: November 27, 2024, 11:14:34 PM by loggie24 »

FabianN

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Re: What causes these types of reconstruction errors?
« Reply #6 on: November 28, 2024, 11:17:02 AM »
Sometimes it also helps to do change the image alignment procedure. F.e. if your images have GPS metadata,  there might be an offset in one pass.  You could try to align images based on estimated locations rather than their source positions in the 'Reference preselection' setting. In case of homogeneous areas that are difficult to match, it might also be worth to try guided image matching.

mraw

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Re: What causes these types of reconstruction errors?
« Reply #7 on: November 28, 2024, 11:50:14 AM »
 @loggie24
If you used the other tools as well....probably it's something different. You could have a look here: https://www.youtube.com/watch?v=zIa_SNz3IuA&t=332s This helped me a lot with the gradual selection tools.

loggie24

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Re: What causes these types of reconstruction errors?
« Reply #8 on: November 28, 2024, 09:05:25 PM »
Sometimes it also helps to do change the image alignment procedure. F.e. if your images have GPS metadata,  there might be an offset in one pass.  You could try to align images based on estimated locations rather than their source positions in the 'Reference preselection' setting. In case of homogeneous areas that are difficult to match, it might also be worth to try guided image matching.
No GPS data here, just handheld shooting. Guided image matching unfortunately doesn't yield any better results. Tried pretty much every alignment option with minimal improvements visible  :'(
« Last Edit: November 28, 2024, 09:11:18 PM by loggie24 »

loggie24

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Re: What causes these types of reconstruction errors?
« Reply #9 on: November 28, 2024, 09:07:50 PM »
@loggie24
If you used the other tools as well....probably it's something different. You could have a look here: https://www.youtube.com/watch?v=zIa_SNz3IuA&t=332s This helped me a lot with the gradual selection tools.
Yeah I already went through this video a while back. At this point I feel like I'v done more or less everything I could. I have gotten some better results masking out the background of the images and applying the mask to the key points on alignment, but any form of post alignment optimization has if anything created worse results.