Cloud to Cloud Registration

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PDMLidar
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Cloud to Cloud Registration

Post by PDMLidar »

Hi All

We conduct MLS as well as static scans. Cloud to Cloud registration in Scene and other packages from static scans works very well however for MLS Cloud to Cloud isn't so good. I have a road that has been segmented during pickup from when I scanned the road instead of scanning the full road and then scanning the side roads, I scanned the side roads as I scanned the main road. The result is a mismatch at the intersections. I have tried using the two methods being create targets on both clouds and match manually and have tried finely adjusting with Cloud Compare with no result. The clouds have either separated or moved no where near where they should be. The tutorials don't help me much either. Does anyone have any suggestions. Thanks in advance
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Re: Cloud to Cloud Registration

Post by 3DForensics »

I've read your post twice and I have to admit that I don't completely understand your description for your scanning workflow. Nonetheless, the bottom line is that if you have enough overlap between the two scans or any two sections of scans, then they should be able to be registered by using Cloud2Cloud. So long as you have enough similarity in the geometry of the overlapping areas, you shouldn't have a problem. The only thing I can think of is that you might be picking through the point cloud onto a point that is different than what you expect. Otherwise, you should be seeing some similarity or proximity between the scans using the point picking method in CloudCompare and the second step would be to use "Finely Register Scans" which is really Cloud2Cloud.

Post some screenshots if you can, I am sure it's something simple.

Eugene
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Re: Cloud to Cloud Registration

Post by PDMLidar »

You right it shouldnt be so hard. Images as attached
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Re: Cloud to Cloud Registration

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Re: Cloud to Cloud Registration

Post by 3DForensics »

So, I can see your first major problem. You have very little geometric structure to align these point clouds. From my perspective, they are basically flat planes and you can't expect a flat plane to align accurately to another flat plane since there are an infinite number of possibilities.

My guess is that your best option is the point picking method and you will be doing a lot of tweaking to get them close. If you had vertical buildings, walls, poles stationary cars...etc., then you would have been a lot better off in my opinion.

Another thing you can try to manually move the point clouds closer using the transform tool and then try to re-register to see if that improves the result. If the point clouds move farther away from one another, the issue is a lack of geometry.

Regards,

Eugene
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Re: Cloud to Cloud Registration

Post by danielgm »

Hi,

Indeed Eugene is right. You need at least some geometry to get a good convergence with the fine registration algorithm (ICP). And the clouds should be already roughly registered. And last but not least be sure to set the 'overlap' parameter correctly (i.e. "how many points of the aligned cloud should actually be used"). Otherwise all the points will be matched and the one that are not overlapping will shift the result.

But in fact with this setup you may encounter another issue: there was a bug in the point pair based alignment tool that could occur when the picked points were all on a plane (which is probably your case). To avoid this bug you can download the latest 2.6.3.beta version (online).
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Re: Cloud to Cloud Registration

Post by PDMLidar »

Thanks for your help. If I just want to sit the two scans on the Z without XY moving do I select the Rotation Z or Translation Z. Thanks Again
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Re: Cloud to Cloud Registration

Post by 3DForensics »

Yes, you can restrict the rotation and also the X, Y or Z translation.
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Re: Cloud to Cloud Registration

Post by danielgm »

If the two clouds are already horizontal, then you can restrict both the rotation (select the "Z" option) and the translation (deselect the Tx and Ty components). This way the cloud will only move along/around the Z axis.
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Re: Cloud to Cloud Registration

Post by PDMLidar »

Hi Daniel,
I have tied restricting to Z only and am finding that the clouds either don't shift or if they do not by much. Surely if the point clouds are overlapping then CC should be able to one to the other. What am I missing
Thanks
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