[pdal] About litree filter: too much computation time, and other questions.

[Ulises Ibarra]

Brad, hello:


El mar, 17 ene 2023 a las 11:46, Bradley Chambers (<brad.chambers at gmail.com>)
escribió:

> Ulises,
>
> What version of PDAL are you using? I think this method will always be
> fairly computationally complex, but we did have a nice speed-up with
> https://github.com/PDAL/PDAL/pull/3520 prior to the 2.4 PDAL release.
>

pdal 2.4.3 (git-version: dcef18)


>
> Radius is distance used to select a dummy point. We default to 100 m, the
> same as is used in the paper. The purpose of the dummy point is to seed the
> class of points that will NOT belong to the tree currently being segmented.
>

Oh, I see that the paper says "We then insert a dummy point n0 that is far
away (e.g., 100 m) from t0 into Ni."

>
> min_points is only used after the current tree has been clustered. If it's
> too small, it is skipped. Changing the min_points will not speed things up
> significantly.
>

 Thanks for the explanation.

>
> I think the biggest opportunities for speeding computation would be to
> downsample the data prior to running the filter or to tile the data into
> smaller chunks. Whether that is a satisfactory solution for your workflow
> or not is really up to you.
>

Please, let me ask you a few questions:

1. Sampling a point inside a cube with a size of about 10 centimeters on
each side, would this seem like a good level of subsampling to you?

2. I could cut the cloud of points into tiles, but would something serious
happen if a tree is half in one tile and half in another?

3. Would each of the files for each tile be processed by batch processing?

Thanks you very much

Ulises


> Brad
>
> On Mon, Jan 16, 2023 at 4:13 PM Ulises Ibarra <
> ulisesmartinibarra at gmail.com> wrote:
>
>> Hello everyone:
>> I have a few questions about the "litree" filter and the computation
>> time.
>>
>> After reading the paper: "2021-Li-PERS.pdf" which talks about the
>> "litree" filter to segment trees, I thought I understood the mechanics of
>> the "litree" filter, but most likely not.
>>
>> When applying filters.litree to a file
>> "L3_0+L3_1_sample_cada_4cm_suelo_dalaunay.las" with 2 million points, the
>> processing time I have so far is a little over 72 hours.
>>
>> My pipeline is the following:
>> {
>> "pipeline":
>> [
>>         {
>>         "type":"readers.las"
>>         },
>>     {
>>         "type":"filters.litree",
>>         "min_points":3000,
>>         "min_height":3.0,
>>         "radius":2.0
>>     },
>>     {
>>         "type":"writers.las",
>>         "minor_version":4,
>>         "scale_x":"0.001",
>>         "scale_y":"0.001",
>>         "scale_z":"0.001",
>>         "extra_dims":"all"
>>     }
>> ]
>> }
>>
>> But I applied previous pipelines to identify where in the process I'm
>> missing:
>>
>> L3_0.las +L3_1.las
>> |
>> (filters.merge) (1)
>> |
>> L3_0+L3_1.las
>> |
>> (filters.sample) (2)
>> |
>> L3_0+L3_1_sample_cada_4cm.las
>> |
>> (filters.elm+filters.outlier+filters.smrf) (3)
>> |
>> L3_0+L3_1_sample_cada_4cm_suelo.las
>> |
>> (filters.hag_delaunay+filters.sort,dimension:HeihtAboveGround,order:DESC)
>> (4)
>> |
>> L3_0+L3_1_sample_cada_4cm_suelo_dalaunay.las (Here is where I am)
>> |
>> (filteres.litree) (5)
>> |
>> L3_0+L3_1_sample_cada_4cm_suelo_dalaunay_litree.las (This is where I want
>> to arrive)
>>
>> *Questions:*
>>
>> 1. What could I be doing wrong?
>> 2. Does the "radius" option refer to the "spacing threshold" they talk
>> about in the paper? I set radios:2. May be I wil  a get a
>> 3. I set the value of min_points to 3000, thinking that any cluster of
>> points better than that value would not be considered and would facilitate
>> the computation. I'm wrong?
>>
>>
>> I hope you can help me. I feel really confused.
>>
>> Tthak you.
>>
>> Ulises M. Ibarra
>>
>>
>> _______________________________________________
>> pdal mailing list
>> pdal at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/pdal
>>
>
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