[pdal] Default 'tt' value when using filters.icp

[Bradley Chambers]

The options are very heavily borrowed from the PCL library, where this was
a squared value. Also, keep in mind that the default values may be better
suited for terrestrial/robotics applications (as opposed to aerial), and
will very likely need some tweaking.

The reported transformation should also be reported in row major order,
like the transformation filter. If you find it is not, I’d consider that a
bug, so feel free to file a ticket.

PRs are always welcome, especially for documentation!

Brad

On Tue, Feb 18, 2020 at 19:11 Joachim Meyer <j.meyer at utah.edu> wrote:

> Hi PDAL-devs,
>
> I have a few questions using `filters.icp` (
> https://pdal.io/stages/filters.icp.html).
> First off, I am very new to aligning two point clouds using this filter
> and have a limited understanding of the underlying algorithm or terminology.
>
> The options give a default value of 9e-8 for the translation threshold.
> Looking at the source code, I read that if I wanted to have a different
> value for that, I should give the square root for a different threshold:
> https://github.com/PDAL/PDAL/blob/a8b0bfbefb7f8ffc467092f11cedc155f862fa6f/filters/IterativeClosestPoint.cpp#L68
>
> Is that correct?
>
> My use case aligns two larger point clouds (100+ million) to each other
> and, to improve performance, I reduced the size of those through
> downsampling in point density, as well as filtering to stable reference
> ground between the two. I save the output of that alignment and now
> planning to use the same matrix with `filters.transformation`. There, it
> says that it should be given in row-major order, and I assume that is what
> the output of `filters.icp` with the —metadata is giving me?
>
> Let me know if I can contribute your answers back to the docs through a PR.
>
>
> Thank you,
>  Joe
>
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