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Author Topic: How to straighten a 'banana' scan in MS Std?  (Read 1461 times)

CheeseAndJamSandwich

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How to straighten a 'banana' scan in MS Std?
« on: December 01, 2021, 11:21:29 AM »
Until now, the models of my underwater scans using gopro timelapse photos has been almost perfect, geometry wise...
I'm now scanning a long site, with long scans at varying depths, and also having 3 x gopros on a 4m pole...  to make it 3x quicker to scan... (works great!)
But now the long scan shows a banana bend in it, bowing up.

Is there any way of correcting this bow in Metashape Standard?  (i.e. i can't use markers, GCPs :'( )

Thanks.
« Last Edit: December 01, 2021, 11:37:48 AM by CheeseAndJamSandwich »

CheeseAndJamSandwich

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #1 on: December 01, 2021, 11:46:36 AM »
Here's some images of a single scan pass (with 3x gopros on the 4m pole) along the reef.
When i combine it with the other scans at different depths (making it wider) it becomes much less bowed, but is still bowed.
I have a full site scan processing, but that might take another 12 hours  :o to finish...  It has an annoying slight bow in it.

SimonBrown

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #2 on: December 01, 2021, 01:11:46 PM »
Curvature on long & thin objects is a common theme. Most are unaware their creation is...bent.

Fixing it might be possible...its not something I have encountered in my own work but here goes:

Constraints - requires Pro
Calibration - take a robust section of another chunk that does not show curvature and export the camera calibration. Then load it into this project. It *might* work. It might not. With calibration having constraints always helps. The problem with the std version is we won't really know how curved, or not, the section is - there is no way of validating what has been created.

Curvature is best avoided by doing long, thin runs.
Agisoft endorsed online Metashape training - see: https://accupixel.co.uk/metashape-training-courses

CheeseAndJamSandwich

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #3 on: December 02, 2021, 07:25:48 AM »
I don't have pro.

How can i straighten a bowed model with Metashape Standard?

Kiesel

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #4 on: December 02, 2021, 09:54:05 AM »
It's a thing of little calibration errors which are adding itself and results to a bended banana model. In standard version the only thing you can do is improving the calibration by improving your shooting pattern for a better on the fly calibration by adding oblique and rolled photos, like you do for lens calibration in Agisoft. Or as Simon Brown suggested you can import a working calibration from another project.


CheeseAndJamSandwich

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #5 on: December 02, 2021, 06:16:14 PM »
It's a thing of little calibration errors which are adding itself and results to a bended banana model. In standard version the only thing you can do is improving the calibration by improving your shooting pattern for a better on the fly calibration by adding oblique and rolled photos, like you do for lens calibration in Agisoft. Or as Simon Brown suggested you can import a working calibration from another project.
As i'm adding more scans, making it wider, it's a lot, lot less bowed up...
And the shape of the reef i'm scanning is a flat section on top, then a shallow angled slope, then a steeper angled slope that continues down deeper... If i widen the scans more to include more of the flat and deeper steeper sections, perhaps it'll come good?

And adding a couple of scan passes at an oblique angle would help straighten it up?
I could add an oblique scan track or two thru the middle of the site where the coral frames are placed, which will actually add more detail to them.  My cameras are normal to the reef usually.

3create

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #6 on: December 02, 2021, 10:33:20 PM »
As already mentioned, oblique and rotated images add a lot to a more solid bundle adjustment, so certainly give that a try.
But it may also really help to calibrate your Gopros in a more controlled "lab-setup" (this doesn't necessaraly mean photos of a calibration pattern).

I've recently used a few GoPros and their lens distortions are very "challenging", but the cams have their use, just like in your project.
The advantage for pre-calibrating Gopros could be that they have a fixed focus, so this parameter will not change.
So you could just try importing the calibration (individually per camera, not just generic per cam model). Of course their will still be some variations in the field (or below water ;) due to temperature differences, general calibration tolerences etc...

Attached a downsampled pic of the UV-distortion map for a Gopro Hero 10: pretty crazy, especially the corners.

CheeseAndJamSandwich

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #7 on: December 03, 2021, 05:45:01 PM »
As already mentioned, oblique and rotated images add a lot to a more solid bundle adjustment, so certainly give that a try.
I'll integrate it into the 'flight plan' for future scans.  It'll actually allow us to add detail thru the coral frame areas.

After adding a few more scans to add width to the site, it came out much better, leaving a small bow... which you can see in the Sketchfab model here:
https://skfb.ly/orxNF

Quote
But it may also really help to calibrate your Gopros in a more controlled "lab-setup" (this doesn't necessaraly mean photos of a calibration pattern).
Gonna have to learn all that... never needed to do it with my other scans, because they're not as long and 'flat'.  I'll follow the tutorials when the next batch of processing is finished...

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I've recently used a few GoPros and their lens distortions are very "challenging", but the cams have their use, just like in your project.
Yep, they're just so convenient for UW, easy to get hold of and fairly capable.  And awesome for filming manta rays on my day off scanning.

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The advantage for pre-calibrating Gopros could be that they have a fixed focus, so this parameter will not change.
So you could just try importing the calibration (individually per camera, not just generic per cam model). Of course their will still be some variations in the field (or below water ;) due to temperature differences, general calibration tolerences etc...
With a more complex topography, that naturally involves a lot of oblique, rotated photos anyway, that comes out looking accurate enough, is there any advantages to doing the pre-calibration?  Does it speed anything up? (remembering that my alignment of 20-30k photos takes 3-4 days)

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Attached a downsampled pic of the UV-distortion map for a Gopro Hero 10: pretty crazy, especially the corners.
Do you have any experience with the 7-8?  How does it compare with the 10?  There's a big jump in mp and a new chip... So i'm assuming it's a lot better...  If only GoPro would sponsor me and give me 6!  :-*
« Last Edit: December 03, 2021, 05:54:37 PM by CheeseAndJamSandwich »

3create

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Re: How to straighten a 'banana' scan in MS Std?
« Reply #8 on: December 04, 2021, 06:15:00 PM »
Quote
...that comes out looking accurate enough, is there any advantages to doing the pre-calibration?
There's no significant speed benefit with pre-calibrated cameras. I haven't used GoPros on any project that requires metric accuracy, so without any measured control points I can't judge the accuracy benefit of pre-calibrating GoPros.
However, pre-calibration for such "challenging" lenses has helped me when Metashape had problems with the initial image alignment (thus adding some constraints for the camera pose calculations).

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Do you have any experience with the 7-8?  How does it compare with the 10?
I've only used the Hero 10, because I was hoping that the new chip would have "good enough" image quality. I'm rather dissapointed with the quality of the stills, and have found the quality of grabbing frames from 5K video footage to be better (thus however loosing the whitebalance correction possibility of the raws).
I tried the Insta 360 One R 1 Inch and it has much better Raw image quality. However, the delay between shots is even worse than GoPro, and one can't remote control several One Rs simultaneously.