100 new default threshold for dense_cloud_max_neighbors tweak to low

[jnb]

Hello,

Working lately on a drone acquisition, I have been stroke by the poor quality of dense cloud noise filtering. I eventually found a topic talking about the same issue : https://www.agisoft.com/forum/index.php?topic=10362.15

The last post from Dieter give me the answer :

Hello

I had the same problems and was amazed at the poor quality of the point cloud in the latest version. Starting with one of the last versions, the dense_cloud_max_neighbors parameter was set to 100 by default.

Experience has shown that this value was too low for me. Especially with objects that were photographed against the sky, extreme deviations formed.

I set this value to -1 and the calculation time was much longer. Now the point clouds are in the quality of the past again.

I hope I could help.

Greeting
Dieter

The default number of depth maps used for the dense cloud filtering stage have indeed been changed  - in June if I am not mistaken - shifting from no limit to 100, without any mention in the change log. To get back to the previous behaviour, you now must use the well known tweak main/dense_cloud_max_neighbors and set it to -1.

This is a major issue for me, as this undocumented change vastly impact the quality of dense cloud on at least some kind of projects. It would have saved me a lot of time and headaches if I have been aware of this change. I understand that this new threshold probably works in most cases but a change so sensitive should have been clearly explained.

You can find attached exemples of how the number of depth maps used impacts this project (default 100, 350 and 850) and the position of the cameras (heavy overlap)

It would be great if in the futur this kind of changes could be more documented.

Regards.

[Alexey Pasumansky]

Hello jnb,

Sorry to hear that the modification of the max_neighbors limit has affected your common workflow. However, we have decided to use this limit due to several factors:
- in most cases the results are not affected, but the processing time is reduced considerably,
- with new depth maps based mesh generation method the dense cloud generation stage can be skipped.
But if the depth maps based approach for some reasons doesn't give you desired results, you can keep using the Tweak setting. The related parameter is not created in GUI since it is not needed in most cases, but may be confusing.

[jnb]

Hello Alexey,

I completely understand that for most projects this limit is enough and why this setting is not in the GUI.
My point was that it would have been better to let us known about this change since it is not a trivial one.

By the way, I indeed work a lot with dense cloud (for profil extraction etc) and also because it allows me to correct imperfections on complex projects before generating the mesh. On simpler cases, the depth maps generated mesh is enough.

[Alexey Pasumansky]

Hello jnb,

Thank you for the explanation.

As a possible workaround, the depth maps based mesh model can be exported to PLY format and then imported back as a dense point cloud. Meshing includes additional depth filtering and the result should be almost noise-free.

Additionally you can use a new Confidence metric for the dense cloud points (implemented in the latest 1.6.0 pre-release) that allows to filter out the points that are generated from the certain number of the depth maps (for example, less than 3).

[jnb]

I have nerver tried this export mesh - import points workaround, thank you for the suggestion.
I have seen the new option in 1.6, looking forward to test it in new projects.

If I can find some time, I'll test this two options on my (very) noisy project and let you know.