Sphere based registration is not accurate, I need some help

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Augusto 3D
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Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

Good morning,
I purchased a set of spheres from a reputable vendor and have been TRYING to use them to the best of our knowledge based on our research and FARO training (when we bought our scanner) and I personally cannot seem to get desirable results.

When using target-based registration, the spheres do not line up to produce round spherical objects, instead, they are off ever so slightly enough for walls and other important features not to align. However, if we forget about target-based registration and run C2C everything lines up (and the spheres are more offset from each other)

I understand that targets can move, however this particular time we used magnet mounts and mounted them to columns and places that would not be moving. The scan resolution was also high to capture plenty of points on the spheres.

The image below is what I am getting - notice how out of alignment the scans are on the round column shown - the same happens with other walls etc.
I am going to use C2C to get this project done since I verified our site measurement matches the results in the point cloud if we go this route.

I m just really annoyed to be honest, that we can't get the spheres and target to work to make things simple, especially considering that this is a proven workflow over the years.

I need some help and If someone feels confident enough and knows the subject in and out to provide some training on spheres, target, and control am happy to pay a reasonable fee for your assistance, send me a DM.

I am also wondering if it could be a scanner issue, maybe we need to do a site compensation - we were advised that we didn't need to go that by Faro since the unit is new and it is site-specific (the scanner is 2 months old). Could there be something wrong with our spheres?

help please, thank you
Screenshot 2022-06-18 152727.jpg
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Leandre Robitaille
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Re: Sphere based registration is not accurate, I need some help

Post by Leandre Robitaille »

To me it could be a series of many factors;
-not enough spheres are used, you need at least 3 between each scans...I run 18 spheres PER scanner, this allows 6+ spheres detected between scans
-bad sphere postionning, you want large triangles, spread them appart within acceptable distances
-wrong sphere diameter is detected, change this in scene.
- mirrored spheres threw windows /false positives not deleted
- i would download taglabs extended toolbox app and delete badly detected spheres, use default values. This will get rid of any yellow spheres...or manually delete the bad spheres using ATS quality manager and open each scans sort by pts number and delete under 80 points.
- bad density used on scanner and spheres arent detected correctly. Outside you will want some 1/2 scans, often 1/4 is enough but outside you want larger sphere spread and 1/2 does help.
1/4 is 14m max for 145mm spheres
1/2 is about 25m max for 145mm spheres
I would not keep anything above 25m for spheres, use large checkerboards above 25m.
- Bad loops. You want to close loops, a long elipse you want spheres /targets in the middle to help balance everything. You can make c2c clusters to help close loops
- sphere moved on site (wind, fences, poles that move, etc)
- bad sphere positionning that hurts algorythm (in grass, threw fences, threw windows)
-scanner is out of calibration

Using targets requires some understanding of the limits of the device and it is threw trial and error you will perfect your on site techniques. Most important thing you want is closing loops, triangles with the targets and a good distance from the scans to spheres.
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

Thank you for the response. You have just validated that my process is no wrong, albeit I didnt use as many spheres but you can still see 3 or 4 and the scans were done at 1/2 -x2 or x4 (I forgot) since I also found that thats the best ROI with our plus series.

The scanner is new so I am not sure that it is out of calibration. I have never done an onsite comp. Is that something that I should have done?

Could the spheres be changing shape with the heat?

PS. What type of large checker boards do you use?
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Re: Sphere based registration is not accurate, I need some help

Post by landmeterbeuckx »

Check the 3d distances from 2 scans where you have the same spheres in it. This already gives some ideas.
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Leandre Robitaille
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Re: Sphere based registration is not accurate, I need some help

Post by Leandre Robitaille »

Augusto 3D wrote: Mon Jun 20, 2022 1:10 am Thank you for the response. You have just validated that my process is no wrong, albeit I didnt use as many spheres but you can still see 3 or 4 and the scans were done at 1/2 -x2 or x4 (I forgot) since I also found that thats the best ROI with our plus series.

The scanner is new so I am not sure that it is out of calibration. I have never done an onsite comp. Is that something that I should have done?

Could the spheres be changing shape with the heat?

PS. What type of large checker boards do you use?
- I would run a accuracy check on the scanner before. This is how you do it ; https://knowledge.faro.com/Hardware/3D_ ... 0the%20bar.
Do the one with ''compare scans taken with different orientation''

-I don't know what spheres you are using, if its the Flexi spheres yes they can change size but it is not noticeble at temperatures human can survive at.

- As for ''large'' targets we had some custom 12'' stickers made here that we put on aluminium plates that we can hang/nail (I also have some that I put on the boxes we use to cary the spheres with), we just asked them to custom print the size https://tdr360.com/en/product-category/ ... r-scanner/
It's very rare we use these but I do find myself using them in those super odd projects where I want a link to close a loop but the shot is far, I do a 1/1 4x focus scan on them and can detect them up to 45ish meters away. Or I just set up the boxes for the spheres at the start around the project and I can detect them every now and then if I need more targets on a scan.
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Re: Sphere based registration is not accurate, I need some help

Post by msivil »

Hi,

If you created the spheres automatically, I would check the sphere fittings visually one by one.
Best Regards,
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

Thanks for all the help guys, I am not too sure what I happnieng. I compared the position on some spheres from different scan locations (quick views) and there are essentially in the same place with less than 1/8" difference. See below:
What is condisered "too much" of a difference?

ST1 ST2
SCAN A -3632.201715 -521.231127 -2053.531601 -2936.501783 -793.515933 -2052.991495
SCAN B -3632.127294 -521.210718 -2053.555923 -2936.541948 -793.826836 -2052.636204

I have a question, if I set anti-correspondence for some spheres and I start to get scans failing, how do I associate them again?
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Re: Sphere based registration is not accurate, I need some help

Post by msivil »

I would recommend checking the sphere fittings visually one by one on 3d view.
Best Regards,
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Leandre Robitaille
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Re: Sphere based registration is not accurate, I need some help

Post by Leandre Robitaille »

Leandre Robitaille wrote: Mon Jun 20, 2022 11:02 am It's very rare we use these but I do find myself using them in those super odd projects where I want a link to close a loop but the shot is far, I do a 1/1 4x focus scan on them and can detect them up to 45ish meters away. Or I just set up the boxes for the spheres at the start around the project and I can detect them every now and then if I need more targets on a scan.
20220620_190935.jpg
Just came back to share my large target solution lol
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Re: Sphere based registration is not accurate, I need some help

Post by INV_Scanning »

After looking at your target tensions I would give it another run without inclinometer in use. Especially for your scan ...001
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