issue with C2C on large airborne lidar lines

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Dimitri
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issue with C2C on large airborne lidar lines

Postby Dimitri » Fri May 19, 2017 3:40 pm

Hi Daniel,

I'm running C2C through command line (but the same issue occurs when using exactly the system in the GUI version) and I have the following issue:
. I'm comparing airborne lidar flight lines (typically with 30-80 millions points) to a reference dataset which does not have the same extent (and may not have in some cases any overlap). 90 % of the time it works perfectly, but for some lines, the calculation basically never ends (and I've waited a few hours).
. In these particular cases, it seems that the approximate distance calculation ends up with very large values (e.g.: average population 27523776 +-27523773), when the typical distance between the 2 point clouds should be smaller. This corresponds to case where there's not a lot of overlap between the two PC, and by randomly picking a few samples, it could be possible to end up with very large distances.
. As a consequence CC picks an octree level of 2 which has 2 consequences (I suppose): (1) it does not maximize the use of CPUs because there's only generally 2 cells; (2) neighbour search is just super long at this low octree level

. Now, I thought that by imposing a min_distance, I would get rid of the problem of the automatic low octree level selection, but it seems that this is not factored in the selection of the octree level, as I always end up with a level 2, while I'm chosing 10 m for instance. Am I right in thinking there's something off here ?

. In the end, I can impose manually the octree level and get the job done very quickly given that I'm note interested in distance greater than 10 m, but because strip lines do not have the same extent, I'm not necesseraly optimal in my choice with respect to the min-distance I'm imposing.

Would it be possible to have the automatic selection of the octree level accounting for the choice of the min-distance to get around these pathological cases ?

Thanks

Dimitri

daniel
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Re: issue with C2C on large airborne lidar lines

Postby daniel » Mon May 22, 2017 5:27 pm

Indeed, I don't think we take the max distance into account when computing the best octree level (If I remember well). And yes, doing the computation at a very low level will be almost equivalent to not using an octree (I shouldn't allow so low levels to be used anyway ;).

I'll see what I can do.
Daniel, CloudCompare admin

Dimitri
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Location: Rennes (France)
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Re: issue with C2C on large airborne lidar lines

Postby Dimitri » Mon May 22, 2017 6:21 pm

No need to hurry, by specifying the octree level I can manage, and I suppose it doesn't happen very often.

Cheers

Dimitri

daniel
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Re: issue with C2C on large airborne lidar lines

Postby daniel » Mon May 22, 2017 6:39 pm

Hum, actually we do take the maximum distance into account...

Do you have an reproducible example that you can share with me?
Daniel, CloudCompare admin

Dimitri
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Joined: Mon Oct 18, 2010 9:01 am
Location: Rennes (France)
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Re: issue with C2C on large airborne lidar lines

Postby Dimitri » Mon May 22, 2017 8:32 pm

Strange.

Ok, I'll prep some data (they tend to be quite big ;-) that reproduces the problem.

Dimitri

daniel
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Re: issue with C2C on large airborne lidar lines

Postby daniel » Mon May 22, 2017 8:55 pm

This must be a tricky case where the heuristic to determine the best level fails (as it's based on several rough assumptions based on the approximate overlap of the clouds and the number of points per cell of the octree at the various subdivision levels).
Daniel, CloudCompare admin


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