[pdal] filters.normal giving odd results

[Bradley Chambers]

Tom,

Glad to see that you found a solution/workaround as posted to SO.

I do believe that we need to add some functionality to the filter to do the
flipping. It's easy math, just need to work out the details of how to
expose this to users. I'll create a ticket for this on GitHub.

Brad

On Thu, Aug 3, 2017 at 3:39 AM Tom van Tilburg <tom.van.tilburg at gmail.com>
wrote:

> Hi,
>
> This question is also on stackexchange:
> https://gis.stackexchange.com/questions/250810/erratic-results-from-filters-normal-in-pdal
> ---
>
> Given this roof structure:
>
> [image: roofpointcloud]
>
> I get the following output for the normalx, after using
>
>     {
>       "type":"filters.range",
>       "limits":"Classification[6:6],ReturnNumber[1:1]"
>     },
>     {
>         "type":"filters.eigenvalues",
>         "knn":8
>     },
>      {
>          "type":"filters.normal",
>          "knn":8
>      },
>
> [image: point normalx]
> (orange to purple : -1 to 1)
>
> Comparable situation for normaly and normalz
>
> Where I was expecting homogeneous colored roofplanes, I get a mixed-up
> situation, where the values appear to be sometimes flipped to the negative.
>
> It seems to me there are some artifacts here based on the laser scan-line,
> considering the flipping of the normal on the same roof plane.
>
> I cannot wrap my head around what happens here. Anybody ideas? The purpose
> of this work is to get the slope and aspect from the roofplanes. I can get
> the slope correct by taking the absolute values of normalz (slope will
> never pass the 90 degrees) but this is not an option for calculating the
> aspect it seems. The resulting pointset can be downloaded here
> <http://research.geodan.nl/sites/tomt/output.las>
>
> Best,
>
>  Tom
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